Add a daily action to test typing-extensions against some downstream users

[JelleZijlstra]

Discussed in #185 (e.g. #185 (comment)).

This would be a daily job that runs the current Git version of typing-extensions against important downstream libraries that rely on typing-extensions. Good candidates would be libraries that rely heavily on runtime type introspection, have test suites that are easy to manage (no complicated build process) and quick to run, and are actively maintained by people who would be able to help if something breaks.

The goal would be to find out about changes in typing-extensions that break these libraries. The outcome of breakages would be that we either patch typing-extensions to fix the issue, or patch the downstream libraries.

Candidates include:

• https://github.com/ilevkivskyi/typing_inspect (maintained by @ilevkivskyi)
• https://github.com/pydantic/pydantic (possible points of contact @samuelcolvin and @adriangb)
• https://github.com/quora/pyanalyze (maintained by me)
• mypy's stubtest tool (maintained by @hauntsaninja)
• I heard something about typed-argument-parse but am not familiar with it
• Maybe beartype?

I'm not planning to work on this myself; contributions welcome! Also feel free to suggest additional libraries that could be useful to include.