Python 3.10 requires to use Py_SET_TYPE() and Py_SET_SIZE() functions
to set an object type or size.

See https://bugs.python.org/issue39573#msg381365 for the context: the incompatible Python 3.10 change was reverted to give time to C extension modules maintainers to port their extension. So here is such change to support Python 3.10 without losing support for old Python versions.

See also https://github.com/pythoncapi/pythoncapi_compat : I used upgrade_python_capi.py script to create this PR, and copied the pythoncapi_compat.h header file from there.

I ran manually the test suite with my patch and it pass on Fedora 33 with Python 3.10.0a2:

python3.10 -m venv ENV
ENV/bin/python setup.py install
ENV/bin/python bitarray/test_bitarray.py 

Without this change, it was not possible to build bitarray on Python 3.10.0a2 (again, the C API incompatible change was reverted in Python 3.10.a3).

Thank you for this PR! I've tested master with the 3.10.a3 release (which reverted this incompatible change), and all works fine. Is this the incompatible C API change expected to be added again in the 3.10 final release?

As I wrote, the change has been reverted in Python 3.10a3, but it may be done again in a future Python version.

See Py_TYPE() documentation:

The Py_SET_SIZE() function must be used to set an object size.

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

Thanks :-)