Unsuppressible mypy syntax error for azure.core when using it from a typechecked project with python 3.9.0

[christophebiocca]

• Package Name: azure.core
• Package Version: 1.9.0
• Operating System: Arch Linux
• Python Version: 3.9.0

Describe the bug
Importing from azure.core.* will cause a hard-to-suppress mypy typechecking error about internal type annotations of azure core that have syntax errors.

I don't understand why it only happens on 3.9.0.

Note that mypy has a really hard time with syntax errors: python/mypy#6897 which makes it hard for the end user to ignore them.

To Reproduce

1. Clone https://github.com/EncircleInc/azure-core-mypy-error
2. pip install -r requirements.lock.txt file.
3. mypy -p test. You'll get something like .../python3.9/site-packages/azure/core/pipeline/policies/_authentication.py:33: error: syntax error in type comment '(TokenCredential, *str, Mapping[str, Any]) -> None
4. Try to suppress the error by adding ignore_errors = True in the mypy.ini file.
5. mypy -p test still errors out.

Expected behavior
No type errors when using default mypy settings, or an error saying there aren't any types available.

[christophebiocca]

Note to anyone else experiencing the issue: a workaround is to add the following to your mypy.ini:

[mypy-azure.core.pipeline.policies._authentication]
follow_imports = skip

A bit of a nuclear option, but nothing short of it will work.

[lmazuel]

That's an interesting bug! PR to fix it #15768