Fix .egg-info metadata support for zipimport

[thatch]

Summary of changes

Previously pkg_resources behavior differed between regular path items and zipimport:

• regular: supports .dist-info and .egg-info metadata
• zipimport: only .dist-info

Extracting an sdist in most cases works for pkg_resources; the change lets you also repack the zip removing the top-level dir (or build with buck/bazel).

Closes #2489

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)

[thatch]

The tests do not run on pytest 6; that one-line change to the version constraint seems necessary, although not directly related to this PR. I didn't see guidance on the order of commits, e.g. whether CI needs to pass on every rev, or you'd rather see a failing test then the fix. Let me know any changes you'd like to see.