Type hint for Distribution.metadata is incorrect

[mpurnell1]

I encountered this while trying to help sentry-python prepare for the changes mentioned in #371. Their linter errored with "PackageMetadata" has no attribute "get", since metadata is advertised to return _meta.PackageMetadata, but actually returns _adapters.Message.

Is this intended, and if so what is the workaroud for the linter?

Reproduction steps:

1. git clone [email protected]:mpurnell1/sentry-python.git
2. pip install tox
3. tox -e linters

[mpurnell1]

It seems the issue is not caused by metadata returning _adapters.Message. Workaround is below, but the typing should maybe still be updated.

... mypy is complaining about PackageMetadata having no .get. Like in most cases that have to do with mypy I'm not sure what's confusing since it isn't the case. Feel free to add a type: ignore[attr-defined] on the line to silence it.

Originally posted by @sentrivana in getsentry/sentry-python#4039 (comment)