Fix inheritance of generic types (PEP-560)

it seems like #3518 did not entirely fix this issue – there will be an error if you try to inherit from the subscripted class.

For example, with the following in example.py, and building the same way as above:

from typing import Generic, TypeVar

T = TypeVar("T")

print(Generic[T])

class MyGeneric(Generic[T]):
  pass

will give:

>>> import example
typing.Generic[~T]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "example.py", line 7, in init example
    class MyGeneric(Generic[T]):
TypeError: __init__() takes 3 positional arguments but 4 were given

_Originally posted by @nikhilmishra000 in https://github.com/cython/cython/issues/2753#issuecomment-616874842_

[da-woods]

@nekokatt As said above, this is most likely a change to the “Py3ClassCreate” utility code: https://github.com/cython/cython/blob/aa8b0d9b89030c0b9ace5698ae5131c623acefa4/Cython/Utility/ObjectHandling.c#L1034 (I’ve linked to the broad section). This C code is largely just copied into each generated Cython file.

We currently don’t handle __mro_entries__ according to PEP-560 so it’s largely a case of working out what stage that should be called at and calling it.

The change will then need some tests, which are probably best done by copying the cpython tests as much as possible. We already have the equivalent file but the MRO tests were omitted since they weren’t yet supported.

[wyfo]

I’ve started to investigate some weeks ago, and I found that this issue was cause by the lack of handling for __mro_entries__ (I would have won some time by better reading https://github.com/cython/cython/issues/2753#issuecomment-616928891) In fact, in Cython, there is no equivalent of __build_class__, where __mro_entries__ is evaluated. That’s why i’m not sure @scoder that a modification of ObjectHandling.c would be enough to add this mecanism in class building.