Replace pgk_resources.parse_version with packaging.version.parse

This removes the final dependency in pkg_resources. Closes #2347.

setup.py

@@ -60,7 +60,8 @@ setup(
         'tornado>=5.0',
         'ghp-import>=1.0',
         'pyyaml_env_tag>=0.1',
-        'importlib_metadata>=3.10'
+        'importlib_metadata>=3.10',
+        'packaging>=20.5'
     ],
     python_requires='>=3.6',
     entry_points={