User-defined protocols

[JukkaL]

This issue tracks the remaining things needed for making user-defined protocols work.

Now that #3132 was merged, mypy understand protocols. However, we still need runtime support for protocols (the Protocol class) and Protocol needs to be added to typeshed before protocols can actually be used in user code.

See also #3132 (comment).

Switching ABCs in typing to be protocols is a separate issue and requires coordination with other users of typeshed.

[JukkaL]

cc @ilevkivskyi

[ilevkivskyi]

OK, here is a PR with runtime implementation in typing_extensions python/typing#464 and a PR with the corresponding stubs python/typeshed#1564

[JukkaL]

This has been supported for a while now.