Support `sys.implementation.name` as feature switch for stubs

[cdce8p]

Problem

Sometimes, attributes and variables are only defined for a specific implementation. E.g. I recently saw that PyPy has a sys.pypy_version_info variable.

At the moment, type checkers only support sys.version_info and sys.platform checks AFAIK.^1

Idea

Add support for sys.implementation.name^2 checks.

import sys

if sys.implementation.name == "pypy":
    pypy_version_info: ...