The next release needs to support Python 3.9

Here's my checklist of what's left:

• PEP 585 support (mostly implement PEP 585 Generic Builtins (Partially solves #7907) #9564)
• PEP 614 testing (Add test for PEP 614 #9759)
• Merge and test new build process for 3.9 wheels (misc: update wheel building and release scripts #9570, mypy_mypyc-wheels: rewrite using Github Actions and cibuildwheel mypyc/mypy_mypyc-wheels#11)
• Add 3.9 to trove classifiers (Add Python 3.9 to trove classifiers #9887)
• Improved documentation of PEP 563 + PEP 585 (Document PEP 585, 563, 604 and more. #9763)
• mypy_extensions support for 3.9 (Add 3.9 to the list of classifiers mypy_extensions#20 / Make a release that supports Python 3.9 mypy_extensions#21)

To clarify the state of support for users stumbling upon this issue, mypy 0.790 will mostly work with Python 3.9.
There are two caveats to running mypy with 3.9:

1. you won't be able to use list[int] syntax without mypy complaining,
2. mypy will be slower since we haven't released mypyc compiled wheels for 3.9.

Note that typeshed in 0.790 has pretty good support for Python 3.9, so using new stdlib functionality should work fine.

PS: if you're getting error: syntax error in type comment with Python 3.9, it's because you're using a really old mypy and you should upgrade to a recent release.