Proof of Concept implementation for the ABC -> Abstract + Virtual split

See, this discuss thread for context. Based on CPython abc.py and _py_abc.py at commit 2b379968.

If the only reason why we can't implement ABC without metaclasses is because it's needed for virtual inheritance, then why don't we start by separating virtual inheritance and abstract method support.

It's not immediately clear to me, why was virtual inheritance bundled together with abstract classes in the first place. The motivating examples for adding virtual inheritance in PEP-3119 seem to me like they should be represented by Protocols rather than Abstract classes. So, it might just be historical happenstance that ABC includes virtual inheritance support rather than an intentional design decision.

So here is my proposal:

1. Extract just the virtual inheritance parts of ABCMeta into a new meta class VirtualMeta. Also provide a helper Virtual class similar to the ABC helper for ABCMeta.

2. Extract just the abstract class parts of ABCMeta (without virtual inheritance support) into a new normal class Abstract (implemented using __init_subclass__).

3. Re-implement ABCMeta and ABC in terms of Abstract, Virtual and VirtualMeta classes, keeping the current behaviour. The new inheritance hierarchy will be an extension of the old hierarchy, so every existing piece of code should continue working.

4. (optional) Emit deprecation warnings whenever someone imports ABC or ABCMeta, suggesting that they should use Abstract/Virtual/VritualMeta directly instead.

With these changes, we would have the following:

• Abstract - a normal class that implements just the abstract base class functionality of ABC (i.e. setting __abstractmethods__). It doesn't use metaclasses. Most people that don't need virtual inheritance can just replace ABC with Abstract.

• VirtualMeta and Virtual implement just the virtual inheritance part of ABC (i.e. register, isinstance and issubclass). As a bonus, people can now create non-abstract virtual inheritance hierarchies.

• ABCMeta inherits from VirtualMeta and automatically adds Abstract and Virtual as base classes in its __new__ method. ABC is still just a helper class that does metaclass=ABCMeta.

Keep in mind, that the current implementation:

• Is written in pure Python. (The C _warning extension loading is disabled)

• Is not exhaustively tested or documented. (There are a few tests and some short docstrings, but it's probably not enough)

• If we choose to deprecate ABC and ABCMeta, we'd need to also update all of the internal uses of ABC/ABCMeta in the standard library, typeshed etc.

Examples

You can run the provided tests with ./run_tests.sh or ./run_tests_in_docker.sh.