Upgrade packaging from 20.8 to 20.9

Run isort over the code base (:issue:`377`)
Add support for the macosx_10_*_universal2 platform tags (:issue:`379`)
Introduce packaging.utils.parse_wheel_filename() and parse_sdist_filename() (:issue:`387` and :issue:`389`)
https://github.com/pypa/packaging/blob/main/CHANGELOG.rst

3rdparty/python/constraints.txt

@@ -15,7 +15,7 @@ mypy==0.800
 mypy-extensions==0.4.3
 packaging==20.9
 pex==2.1.28
-pip==20.2.3
+pip==20.2.1
 pluggy==0.13.1
 psutil==5.7.0
 py==1.10.0