Implement PEP-560: core support for generic types

Seems that Generic.__class_getitem__ can’t be called automatically after cythonization.

Code sample example.py:

from typing import Generic, TypeVar

T = TypeVar("T")
print(Generic[T])

Cythonization: cythonize -a -i example.py

The cythonized code fails with TypeError:

>>> import example
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "example.py", line 5, in init example
    print(Generic[T])
TypeError: 'type' object is not subscriptable

Cython 0.29.1 Python 3.7.1

[scoder]

See https://github.com/cython/cython/issues/2753#issuecomment-450492882 above. PR welcome.

[gst]

I confirm the workaround works… it’s not very nice but it works. Thanks for it 😃