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.name² checks.

import sys

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

Footnotes

1. https://mypy.readthedocs.io/en/stable/common_issues.html?highlight=sys.platform#python-version-and-system-platform-checks ↩

2. https://peps.python.org/pep-0421/#required-attributes ↩