Need more type definitions

[jaraco]

In this twine commit, I learned that twine in unsatisfied with the type declarations and needs more hand holding.

[bhrutledge]

My general interpretation is that importlib_metadata has a lax mypy configuration:

importlib_metadata/mypy.ini

Lines 1 to 2 in aa9f799

	[mypy]
	ignore_missing_imports = True

which could cause errors in any project that has a more strict configuration (like Twine). Since this is a core library, putting the effort into satisfying a strict mypy configuration could bring value to lots of projects.

[janosh]

satisfying a strict mypy configuration could bring value to lots of projects.

Sounds like @asottile would be exactly the right guy for the job. 😄

[jaraco]

This issue is open-ended and partially-complete. I'm closing it, but let it be known that I welcome improvements to the type declarations, especially those improvements that add clarity and information that would otherwise be undiscoverable or difficult to discover.