Add test coverage monitoring

[JelleZijlstra]

Getting typing-extensions to 100% test coverage seems like a realistic and useful goal. Doing this ensures that we have thorough tests and also gives more confidence when adding new code.

Contributions towards this goal are welcome. We should start with running a coverage tool to get a sense of how much coverage we're currently missing, then add tests to cover the missing bits, then set up CI so that it enforces 100% coverage. A complication might be that lots of code is version-specific: coverage computation should ideally sum across all supported versions of Python.

[AlexWaygood]

The covdefaults coverage plugin might be useful for us. It allows you to suppress coverage calculations on a version branch only if you're running a specific Python version: https://github.com/asottile/covdefaults?tab=readme-ov-file#version-specific--pragma-no-cover