Use Literal overloads to give better types to subprocess (#3110)

This gives better types to `subprocess.check_output` and `subprocess.run`
by laboriously overloading using literals.

To support `run`, I turned `CompletedProcess` into `_CompletedProcess[T]`
with `CompletedProcess = _CompletedProcess[Any]`. I could pretty easily
be convinced that it would be better to just make `CompletedProcess`
generic, though.

I'd like to do the same for Popen but need to make mypy support
believing the type of `__new__` in order for that to work.

Author: msullivan