[script] Add script to help generate the Changelog (#23744)

## Summary & Motivation

As title. This adds a script to read through the commits since the
previous release and create changelog entries for them. The expected
workflow is for PR authors to add their changelog entries in their
commit messages, i.e.:

CHANGELOG:
Added internal script to generate changelog entries.

In the case that your PR does not actually require any acknowledgement
in the changelog (such as this one, as it is an internal script), you
can indicate that by adding the NOCHANGELOG string anywhere in your
commit message.

If neither of these strings are found, then the commit is considered
"Undocumented", and manual intervention will be required. We can trial
this workflow for a few weeks before adding CI hooks to enforce that you
add changelog text to your commits.

To generate a new changelog, you can run:

```
python scripts/create_changelog.py 1.8.2 | pbcopy
```

Which you can then paste into your collaborative docs editor of choice
(note, once we use CI to enforce CL entries, we can cut this manual step
out entirely).


## How I Tested These Changes

Check out this sample changelog doc generated by the script:
https://www.notion.so/dagster/7dd240b804b5439c8e65725d24ac0f29?showMoveTo=true&saveParent=true

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,7 @@
 ## Summary & Motivation
 
 ## How I Tested These Changes
+
+## Changelog [New | Bug | Docs]
+
+> Replace this message with a changelog entry, or `NOCHANGELOG`

scripts/generate_changelog.py

@@ -0,0 +1,165 @@
+import os
+import re
+import sys
+from collections import defaultdict
+from typing import Iterator, List, Mapping, NamedTuple, Optional, Sequence
+
+import click
+import git
+from path import Path
+
+GITHUB_URL = "https://github.com/dagster-io/dagster"
+OSS_REPO = git.Repo(Path(__file__).parent.parent)
+INTERNAL_REPO = git.Repo(os.environ["DAGSTER_INTERNAL_GIT_REPO_DIR"])
+
+CHANGELOG_HEADER_PATTERN = re.compile(r"## CHANGELOG.*\[\s*(.*?)\s*\]")
+IGNORE_TOKEN = "NOCHANGELOG"
+
+CATEGORIES = {
+    "NEW": "New",
+    "BUG": "Bugfixes",
+    "DOCS": "Documentation",
+    "BREAKING": "Breaking Changes",
+    "DEPRECATE": "Deprecations",
+    "PLUS": "Dagster Plus",
+    None: "Invalid",
+}
+
+
+class ParsedCommit(NamedTuple):
+    issue_link: str
+    changelog_category: str
+    raw_changelog_entry: Optional[str]
+    raw_title: str
+    author: str
+    repo_name: str
+
+    @property
+    def documented(self) -> bool:
+        return bool(self.raw_changelog_entry)
+
+
+def _get_previous_version(new_version: str) -> str:
+    split = new_version.split(".")
+    previous_patch = int(split[-1]) - 1
+    assert previous_patch >= 0, "Must explicitly set `previous_version` on major releases."
+    return ".".join([*split[:-1], str(previous_patch)])
+
+
+def _get_libraries_version(new_version: str) -> str:
+    split = new_version.split(".")
+    new_minor = int(split[1]) + 16
+    return ".".join([split[0], str(new_minor), split[2]])
+
+
+def _get_parsed_commit(commit: git.Commit) -> ParsedCommit:
+    """Extracts a set of useful information from the raw commit message."""
+    title = str(commit.message).splitlines()[0]
+    # me avoiding regex -- titles are formatted as "Lorem ipsum ... (#12345)" so we can just search
+    # for the last octothorpe and chop off the closing paren
+    issue_number = title.split("#")[-1][:-1]
+    issue_link = f"[#{issue_number}]({GITHUB_URL}/pull/{issue_number})"
+
+    # find the first line that has `CHANGELOG` in the first few characters, then take the next
+    # non-empty line
+    found = False
+    changelog_category = "Invalid"
+    raw_changelog_entry = None
+    for line in str(commit.message).split():
+        if found and line:
+            raw_changelog_entry = line
+            break
+        # give a buffer to allow us to match formats such as "## Changelog"
+        match = CHANGELOG_HEADER_PATTERN.match(line)
+        if match:
+            changelog_category = CATEGORIES.get(match.group(1), changelog_category)
+            found = True
+
+    return ParsedCommit(
+        issue_link=issue_link,
+        changelog_category=changelog_category,
+        raw_changelog_entry=raw_changelog_entry,
+        raw_title=title,
+        author=str(commit.author.name),
+        repo_name=str(commit.repo.git_dir).split("/")[-2],
+    )
+
+
+def _normalize(name: str) -> str:
+    return name.replace(" ", "").lower()
+
+
+def _is_external_commit(commit: git.Commit) -> bool:
+    # not super accurate at the moment, we'll probably need to actually ping the Github API
+    return bool(commit.co_authors) and any(
+        _normalize(str(a.name)) != _normalize(str(commit.author.name)) for a in commit.co_authors
+    )
+
+
+def _get_documented_section(documented: Sequence[ParsedCommit]) -> str:
+    grouped_commits: Mapping[str, List[ParsedCommit]] = defaultdict(list)
+    for commit in documented:
+        grouped_commits[commit.author].append(commit)
+
+    documented_text = ""
+    for category in CATEGORIES.values():
+        documented_text += f"### {category}\n\n"
+        for commit in grouped_commits.get(category, []):
+            documented_text += f"\n* {commit.issue_link} {commit.raw_changelog_entry}"
+    return documented_text
+
+
+def _get_undocumented_section(undocumented: Sequence[ParsedCommit]) -> str:
+    undocumented_text = "# Undocumented Changes"
+
+    grouped_commits: Mapping[str, List[ParsedCommit]] = defaultdict(list)
+    for commit in undocumented:
+        grouped_commits[commit.author].append(commit)
+
+    for author, commits in sorted(grouped_commits.items()):
+        undocumented_text += f"\n- [ ] {author}"
+        for commit in commits:
+            undocumented_text += (
+                f"\n\t- [ ] (repo:{commit.repo_name}) {commit.issue_link} {commit.raw_title}"
+            )
+    return undocumented_text
+
+
+def _get_commits(
+    repos: Sequence[git.Repo], new_version: str, prev_version: str
+) -> Iterator[ParsedCommit]:
+    for repo in repos:
+        for commit in repo.iter_commits(rev=f"release-{prev_version}..release-{new_version}"):
+            if IGNORE_TOKEN in str(commit.message):
+                continue
+
+            yield _get_parsed_commit(commit)
+
+
+def _generate_changelog(new_version: str, prev_version: str) -> None:
+    documented: List[ParsedCommit] = []
+    undocumented: List[ParsedCommit] = []
+
+    for commit in _get_commits([OSS_REPO, INTERNAL_REPO], new_version, prev_version):
+        if commit.documented:
+            documented.append(commit)
+        else:
+            undocumented.append(commit)
+
+    header = f"# Changelog {new_version}\n\n## {new_version} (core) / {_get_libraries_version(new_version)} (libraries)\n\n"
+    sys.stdout.write(
+        f"{header}\n{_get_documented_section(documented)}\n{_get_undocumented_section(undocumented)}"
+    )
+
+
+@click.command()
+@click.argument("new_version", type=str, required=True)
+@click.argument("prev_version", type=str, required=False)
+def generate_changelog(new_version: str, prev_version: Optional[str] = None) -> None:
+    if prev_version is None:
+        prev_version = _get_previous_version(new_version)
+    _generate_changelog(new_version, prev_version)
+
+
+if __name__ == "__main__":
+    generate_changelog()