Add typing to Stacky (#15)

Summary

As indicated in the title, the idea is to have things more strongly
typed so that we can run mypy on diff to ensure that we have less risk
of regressing.

Testing

Ran:
```
mypy src
```

Author: mpatou

pyproject.toml

@@ -0,0 +1,8 @@
+[tool.black]
+line-length = 120
+target-version = ['py310']
+include = '\.pyi?$'
+
+[tool.isort]
+profile = "black"
+line_length = 120

setup.py

@@ -1,28 +1,29 @@
-from setuptools import setup, find_packages
 import pathlib
 
+from setuptools import find_packages, setup
+
 here = pathlib.Path(__file__).parent.resolve()
 
 # Get the long description from the README file
 long_description = (here / "README.md").read_text(encoding="utf-8")
 
 setup(
-    name="rockset-stacky",  
-    version="1.0.10",  
+    name="rockset-stacky",
+    version="1.0.11",
     description="""
     stacky is a tool to manage stacks of PRs. This allows developers to easily 
     manage many smaller, more targeted PRs that depend on each other.
-    """, 
-    long_description=long_description,  
-    long_description_content_type="text/markdown", 
-    url="https://github.com/rockset/stacky", 
-    author="Rockset", 
-    author_email="[email protected]", 
+    """,
+    long_description=long_description,
+    long_description_content_type="text/markdown",
+    url="https://github.com/rockset/stacky",
+    author="Rockset",
+    author_email="[email protected]",
     keywords="github, stack, pr, pull request",
-    package_dir={"": "src"}, 
+    package_dir={"": "src"},
     packages=find_packages(where="src"),
     python_requires=">=3.8, <4",
-    install_requires=["asciitree", "ansicolors", "simple-term-menu"],  
+    install_requires=["asciitree", "ansicolors", "simple-term-menu"],
     entry_points={
         "console_scripts": [
             "stacky=stacky:main",
@@ -32,4 +33,4 @@
         "Bug Reports": "https://github.com/rockset/stacky/issues",
         "Source": "https://github.com/rockset/stacky",
     },
-)
+)