Added a py.typed so mypy prefers us to typeshed

[alex]

closes #5796

[intgr]

Note that this change regresses the precision of type checkers significantly, at least in my use case.

See for example https://github.com/pyca/cryptography/blob/b24d522b05b2220fbcb74363e454c2997a0c7ab7/src/cryptography/x509/extensions.py, there are almost no type hints in this file.

Compare that to the typeshed version https://github.com/python/typeshed/blob/master/stubs/cryptography/cryptography/x509/__init__.pyi

Note that a dozen or so people contributed to the cryptography stubs in typeshed, me included. I think cryptography upstream should be more considerate that this work isn't lost at a whim.

But now that this is done, what are the options going forward? I can only think of

• (1) revert addition of py.typed in cryptography
• (2) import hints from typeshed

Unfortunately option (2) seems like significant work and not a short-term solution. I understand that (1) isn't great either. Any other options that I didn't think of?

[reaperhulk]

@intgr Thanks for pointing out that we missed this! We've started the typing work on that module in #5812 and will get it done ASAP. If there are other areas we've missed please open a new issue so we can track it!