Mypy 0.750 not compatible with Python 3.9

[thundergolfer]

At time of writing, the README offers Mypy version 0.750 in it's demo code/config. It can be found that this version is not compatible with Python 3.9, producing a great number of "syntax error in type comment" errors when attempting to run a type-checking aspect or rule. See this issue: python/mypy#9916.

The investigation of this issue was done in the following reproduction repo: github.com/bogdan-lytvynov/bazel-mypy-integration-failed

If you clone that repo and attempt to run the integration using Python 3.9, with --test_output=all you will see the following kind of error lines:

/usr/local/lib/python3.9/site-packages/mypy/typeshed/stdlib/2and3/builtins.pyi:41: error: syntax error in type comment
/usr/local/lib/python3.9/site-packages/mypy/typeshed/stdlib/2and3/builtins.pyi:42: error: syntax error in type comment
/usr/local/lib/python3.9/site-packages/mypy/typeshed/stdlib/2and3/builtins.pyi:43: error: syntax error in type comment
/usr/local/lib/python3.9/site-packages/mypy/typeshed/stdlib/2and3/builtins.pyi:46: error: syntax error in type comment
...

To see that the error is not coming from the Bazel integration, I did python3 -m pip install mypy==0.750 and ran mypy print/print.py and got the same errors.

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Action:

This is not a bug in the integration but I'll add a note in the README about it.