Add stub for subprocess.list2cmdline function

[miggaiowski]

Typeshed doesn't have this function, and it's not even documented. Found this about it: https://github.com/python/cpython/blob/master/Lib/subprocess.py#L161

Code using this list2cmdline made mypy complain.

[gvanrossum]

The comment suggests that it's intentionally undocumented. I don't see a reason to sanction its use in typeshed. If you want it public you're going to have to petition on bugs.python.org for that first.

[miggaiowski]

What if we add a comment here saying this is not sanctioning its use, but merely fixing typing errors for code already using it.
This function is undocumented, but it's not marked as private that shouldn't be used externally (there are mentions about it in the documentation).

[gvanrossum]

It is clearly meant to be private. The comment you linked to states it is an implementation detail. I really don't want this in the stubs. You can put a # type: ignore on it if you really feel your code is correct.

[miggaiowski]

After reading the issue referencing this PR, should I update it with a comment and reopen this?

[gvanrossum]

Sure, we'll reconsider the PR.

[gvanrossum]

Thanks!