Implement PEP-560: core support for generic types

[verailina]

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]

PEP-560 is not currently implemented. Probably easy to do, but needs to be done.
https://www.python.org/dev/peps/pep-0560/

[scoder]

This should be relatively easy.

• copy over the tests from CPython
• run the tests to see what C code gets generated currently
• copy over parts of the implementation from CPython, drop them in the right places according to the C code generated for the tests (probably just the part from CPython's abstract.c)
• adapt and optimise.

PR welcome.

[msg555]

For whatever it's worth I'm still running into apparently this issue with Cython 0.29.16/Python 3.7.7 running the same test as the OP.

[scoder]

See #2753 (comment) above. PR welcome.