Add a `collection.abc.Function` ABC for function-like objects

Feature or enhancement

Proposal:

In the Cython project, we generate a function type that is mostly compatible with Python's function type. There is currently no (simple/reasonably fast) way to detect this kind of object, but there are use cases and Cython aware projects have found their workarounds in one way or another. But it still means that all code that wants to specially support both Python and Cython functions in the same (or similar) way needs to do something special to detect both, rather than just saying "is this a function?".

There should be an abc.Function ABC that other function implementations can register with.
That would make the detection a simple and straight forward isinstance(obj, abc.Function).

inspect.isfunction() does not help since it specifically tests that the object is a Python function object, which is good to have, simple and fills a need. The ABC is for use cases where code needs to test whether an object is a callable with a function-like interface, e.g. for introspection, signature, code-like object, etc., however complete that implementation then is.

This is probably also relevant for other tools like mypyc or numba that provide binary functions to Python in one way or another.

I'd also add this ABC to the PyPI package backports_abc which provides missing ABCs for older Pythons.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

Make inspect.isfunction() support Cython functions:
https://github.com/cython/cython/issues/6379

Some code trying to detect Cython functions:

• https://github.com/vmware-archive/pop/blob/f3a67f913ee92cf855889719a23f662dd435f39d/pop/loader.py#L222
• https://github.com/sphinx-doc/sphinx/blob/a6d7ae16739bf92a032a7c4df0297db7cf120ec9/sphinx/util/inspect.py#L342-L347
• https://github.com/quora/qcore/blob/f93ccc0f04858f1c606d3b95f05ff315f4fd47bf/qcore/inspection.py#L151-L163

Start by reviewing the linked Cython issue and the three example implementations that detect Cython functions. No CPython file or test is named, so the first step is locating the relevant ABC and introspection areas and confirming the proposed API with maintainers. Done requires an accepted design for the new ABC, its registrations and tests, plus the corresponding backports_abc change.