Use Literal overloads to give better types to subprocess

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.