docs: Use importlib.metadata to get version info (#2477)

* pkg_resources is deprecated as an API, so stop using it.
   - c.f. https://setuptools.pypa.io/en/latest/pkg_resources.html
* importlib.metadata.version is the recommended way to get package
  version information.
   - c.f. https://docs.python.org/3/library/importlib.metadata.html

docs/conf.py

@@ -15,11 +15,11 @@
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use Path('../relative_path_to_dir').resolve() to make it absolute, like shown here.
 
+import importlib.metadata
 import sys
 from pathlib import Path
 
 import jupytext
-from pkg_resources import get_distribution
 
 sys.path.insert(0, str(Path('./exts').resolve()))
 
@@ -123,7 +123,7 @@ def setup(app):
 # |version| and |release|, also used in various other places throughout the
 # built documents.
 # The full version, including alpha/beta/rc tags.
-release = get_distribution('pyhf').version
+release = importlib.metadata.version("pyhf")
 # for example take major/minor/patch
 version = '.'.join(release.split('.')[:3])