Script and CI action to validate backwards compatibility

[amyreese]

Adds a simple script that parses the PyPI JSON API for public (non-yanked) releases
of usort >= 1.0, and then checks those old versions against sorting from the
current/local version, and passes only if all of those previous versions produce
no changes.

Example output:

$ make backcompat
python check_backcompat.py
discovered versions 1.0.4, 1.0.2, 1.0.1, 1.0.0

sorting with local version ...
usort, version 1.0.3.dev47+g6cc5725.d20220913
done

checking version 1.0.4 ...
usort, version 1.0.4
clean

checking version 1.0.2 ...
usort, version 1.0.2
clean

checking version 1.0.1 ...
usort, version 1.0.1
clean

checking version 1.0.0 ...
usort, version 1.0.0
clean

success!