gh-126705: Make os.PathLike more like a protocol

[tungol]

This MR makes os.PathLike more protocol-like in two ways:

1. adds it to typing._PROTO_ALLOWLIST so it can be used as a base class in other protocols
2. use class PathLike(metaclass=abc.ABCMeta) instead of class PathLike(abc.ABC). This aligns with the other protocol-like ABCs in collections.abc. Actual protocols do have that metaclass but don't have the abc.ABC base class. Doing this makes it a little smoother for typeshed to define the class as class PathLike(Protocol) in the stubs.

• Issue: Make os.Pathlike more similar to a Protocol #126705

[bswck]

On-topic: LGTM
Off-topic:
Makes me wonder why not allow ABC in bases here:

cpython/Lib/typing.py

Line 2006 in a6d48e8

base in {object, Generic}

It's such a common base class, is there anything mutually exclusive with abstract classes and protocols? This PR proves false.

[tungol]

I think the need for the explicit allowlist is that not all ABCs are protocol-like; only ABCs with a __subclass_hook__ that's properly constructed for that purpose should be mixed with protocols. It's not that they're mutually exclusive, it's just that most of them aren't suitable.

[tungol]

I think I see the confusion: being able to use PathLike as a protocol base only requires adding it to _PROTO_ALLOWLIST. Adjusting the inheritance of the class is not required for that functionality, it just makes it a little cleaner. Of my two points, number 1 can be implemented without number 2 (or vice versa).

[asvetlov]

Looks good, thanks!