Provide mechanism to check distribution config

[ross-spencer]

Adds a package-check command to make sure the package description for pypi is valid.

Both new combinations of make recipe work as anticipated:

twine check dist/*
Checking distribution dist/metsrw-0.3.12-py2.py3-none-any.whl: Passed
Checking distribution dist/metsrw-0.3.12.tar.gz: Passed
twine upload dist/* --repository-url https://upload.pypi.org/legacy/
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Connected to archivematica/Issues#875

[cole]

Looks good to me, just the failing CI checks to take care of. Black wants double quotes for strings. I'll approve, and if you can fix the linting failure before merging that'd be grand.

It looks like the other failure is we don't have a pinned version of lxml and they have dropped 3.4 support. I think to solve that we should probably drop 3.4 support (it's no longer supported), but we can also avoid future failures like this by pinning the versions. That is a separate issue though, and I feel like this PR can be merged without passing that check.

[ross-spencer]

Less haste, more speed! Thanks Cole. Done!