importlib_metadata: define protocol for Distribution.metadata

[FFY00]

Signed-off-by: Filipe Laíns lains@riseup.net

[FFY00]

Resolves #126.

[FFY00]

We run into the case here where we have different coverage in different versions, because of the backport import that will only run on older Python. The proper solution would be to run tests for both versions and merge the coverage, but since it is just an import, I added a # pragma: no cover there.

[FFY00]

Btw, relevant typeshed definitions: https://github.com/python/typeshed/blob/master/stdlib/3/email/message.pyi

[jaraco]

This looks great. I'll add a changelog entry and get it out. Thanks!

[jaraco]

Well drat. I renamed the default branch from master to main and now diffcov is failing again as it was in #257, despite the fix still being applied. It seems the name 'master' is cached or hard-coded in diff-cover.

[jaraco]

I'm going to proceed with manually merging and ignore the diff-cover failure for now. If the issue persists in new PRs, I'll probably just get rid of that footgun.