As a description of the idea I'm attaching a rough patch with an implementation. Some tests fail because of the following:

1) I didn't find a good place to register the Buffer ABC for mmap.mmap, and array.mmap; the modeules themselves are C extensions (and it's possible but weird to register the ABC from the C API); I could import those in typing.py but then I'd make typing depend from mmap and array which also feels wrong; so I didn't do that and the related tests that I added fail.

2) I had a similar situation with io.BytesIO. In that case there's a python module and I'm doing the abc.register there, but now io imports typing, and the test_site fails because many new modules (typing and its dependencies) are imported on startup. The alternative is to make typing depend on io (in this case it could be a reasonable idea)

After these, I'm feeling that typing.py may not be the best place for this (but given that collections.abc isn't either, I'm not sure where's the right place)