[psutil] Add cache_clear to psutil.process_iter typing

[emmanuel-ferdman]

PR Summary

This PR adds typing support for the cache_clear method of psutil.process_iter. The fix replaces the function declaration with a _ProcessIterCallable Protocol that defines both the original callable signature and the new cache_clear() method, resolving mypy errors when accessing psutil.process_iter.cache_clear() while maintaining full backward compatibility with existing code.

Closes #14281.

[srittau]

Thanks, looks mostly good, a few specific notes below.

Generally regarding tests, we try to use a "less is more" approach in typeshed (as opposed to your typical unit tests in other projects), as the tests are fairly expensive, but are also often just repeating the type annotations, and many typical problems are already caught by tools like stubtest.

In this specific PR, it definitely makes sense to test that process_iter is still a callable, and that the clear_method() function exists as an attribute and is callable, but we don't need to the different argument permutations of process_iter(), and the last test just seems redundant with prior tests.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉