Port mypyc to Python 3.11: use Py_SET_SIZE()

[vstinner]

"Py_TYPE(t) = metaclass;" fails with a compiler error on Python 3.11:

• https://docs.python.org/dev/whatsnew/3.11.html#c-api-changes

• https://docs.python.org/dev/c-api/structures.html#c.Py_TYPE

• Replace "Py_TYPE(t) = metaclass;" with "Py_SET_TYPE(t, metaclass);"

• Replace "Py_SIZE(self) = newsize;" with "Py_SET_SIZE(self, newsize);"

• Add a copy of pythoncapi_compat.h header to add Py_SET_TYPE() and
  Py_SET_SIZE() functions to Python 3.8 and older:
  https://github.com/pythoncapi/pythoncapi_compat

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Description

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Test Plan

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[vstinner]

I'm the author of the Py_SIZE() change in Python 3.11 :-) It has been approved by the Steering Council: python/steering-council#79

I'm also working on a PEP which changes other macros and explains the rationale for the Py_SIZE() change, see: https://www.python.org/dev/peps/pep-0674/

Moreover, I'm the author of the pythoncapi_compat project and its pythoncapi_compat.h header file: https://github.com/pythoncapi/pythoncapi_compat I'm trying to propose it as a standard way to support multiple Python versions with a single C code base. The header provides new C API functions on old Python versions.

[vstinner]

Right now, I cannot run runtests.py with Python 3.11 because python3 -m pip install -r test-requirements.txt fails on installing typed-ast:

    gcc -pthread -Wno-unused-result -Wsign-compare -g -Og -Wall -O0 -fPIC -Iast27/Include -I/home/vstinner/dev/mypy/env/include -I/home/vstinner/python/main/Include -I/home/vstinner/python/main -c ast27/Python/mystrtoul.c -o build/temp.linux-x86_64-3.11-pydebug/ast27/Python/mystrtoul.o
    ast27/Python/mystrtoul.c: In function ‘Ta27OS_strtoul’:
    ast27/Python/mystrtoul.c:112:21: error: ‘_PyLong_DigitValue’ undeclared (first use in this function)
      112 |                 if (_PyLong_DigitValue[Py_CHARMASK(str[1])] >= 16) {
          |                     ^~~~~~~~~~~~~~~~~~

[vstinner]

Right now, I cannot run runtests.py with Python 3.11 because python3 -m pip install -r test-requirements.txt fails on installing typed-ast:

Issue already reported upstream: python/typed_ast#167

[vstinner]

I also got build issues when trying to install the lxml dependency. Cython 0.29.24 is not compatible with Python 3.11 yet: the cython/cython@e138a84 change (part of the 0.29.x branch) is needed, but there is no Cython 0.29.x release including this change yet. I asked for a Cython release at cython/cython#4465

[msullivan]

This seems great, thank you for the PR. I'm not too worried about not running the 3.11 tests.

[vstinner]

This seems great, thank you for the PR. I'm not too worried about not running the 3.11 tests.

Thanks for the merge ;-)