Run pyupgrade on the source code

[sobolevn]

I ran pyupgrade on all mypy/* files.
Command: pyupgrade --py37-plus mypy/**/*.py, but I had to do it twice, because pyugrade changes %s to .format first, then to f strings.

Basically, it does several things:

1. Normalizes typing imports
2. Uses f strings where possible
3. Uses different subprocess.run signature

Refs #12711

I will address pyinfo.py in a separate PR.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[sobolevn]

Thanks! I will also request @hauntsaninja's review as someone who has already done that 🙂