Port to Python 3.11: use Py_SET_SIZE()

[vstinner]

On Python 3.11, "Py_SIZE(str) = actual_size;" fails with a compiler
error. Since Python 3.9, Py_SET_SIZE() must be used instead:

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

Add a copy of the pythoncapi_compat.h header file to get
Py_SET_SIZE() on Python 3.8 and older:
https://github.com/pythoncapi/pythoncapi_compat

[martindurant]

I am no expert, so hopefully someone else can confirm this is good. If there are no objections, I can merge it in a couple of days.

[vstinner]

Without my change, the build fails:

$ git switch master
$ rm -rf build; LANG= ~/python/main/python setup.py build_ext
(...)
building 'snappy._snappy' extension
(...)
gcc -pthread -Wno-unused-result -Wsign-compare -g -Og -Wall -O0 -fPIC -I/home/vstinner/python/main/Include -I/home/vstinner/python/main -c src/snappy/crc32c.c -o build/temp.linux-x86_64-3.11-pydebug/src/snappy/crc32c.o
gcc -pthread -Wno-unused-result -Wsign-compare -g -Og -Wall -O0 -fPIC -I/home/vstinner/python/main/Include -I/home/vstinner/python/main -c src/snappy/snappymodule.cc -o build/temp.linux-x86_64-3.11-pydebug/src/snappy/snappymodule.o
In file included from /home/vstinner/python/main/Include/Python.h:42,
                 from src/snappy/snappymodule.cc:29:
src/snappy/snappymodule.cc: In function ‘PyObject* maybe_resize(PyObject*, size_t, size_t)’:
/home/vstinner/python/main/Include/object.h:146:29: error: lvalue required as left operand of assignment
  146 | #define Py_SIZE(ob) _Py_SIZE(_PyVarObject_CAST_CONST(ob))
      |                     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/snappy/snappymodule.cc:72:13: note: in expansion of macro ‘Py_SIZE’
   72 |             Py_SIZE(str) = actual_size;
      |             ^~~~~~~
error: command '/usr/bin/gcc' failed with exit code 1

With the change, the build completes succesfully:

$ git switch python311 
$ rm -rf build; LANG= ~/python/main/python setup.py build_ext
(...)
building 'snappy._snappy' extension
(...)
gcc -pthread -Wno-unused-result -Wsign-compare -g -Og -Wall -O0 -fPIC -I/home/vstinner/python/main/Include -I/home/vstinner/python/main -c src/snappy/crc32c.c -o build/temp.linux-x86_64-3.11-pydebug/src/snappy/crc32c.o
gcc -pthread -Wno-unused-result -Wsign-compare -g -Og -Wall -O0 -fPIC -I/home/vstinner/python/main/Include -I/home/vstinner/python/main -c src/snappy/snappymodule.cc -o build/temp.linux-x86_64-3.11-pydebug/src/snappy/snappymodule.o
(...)
g++ -pthread -shared build/temp.linux-x86_64-3.11-pydebug/src/snappy/crc32c.o build/temp.linux-x86_64-3.11-pydebug/src/snappy/snappymodule.o -lsnappy -o build/lib.linux-x86_64-3.11-pydebug/snappy/_snappy.cpython-311d-x86_64-linux-gnu.so

[vstinner]

Nice, thanks ;-)