gh-146636: PEP 803: add Py_TARGET_ABI3T and .abi3t.so extension

[encukou]

• Issue: Implement PEP 803 -- abi3t #146636

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit db32325 🤖

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[vstinner]

LGTM.

Please document changes in the commit message:

• Add Py_TARGET_ABI3T macro.
• Add ".abi3t.so" to importlib EXTENSION_SUFFIXES.
• Remove ".abi3.so" from importlib EXTENSION_SUFFIXES on Free Threading.

[vstinner]

I tested manually building test_cext with Py_TARGET_ABI3T and rename the extension to test_cext.abi3t.so:

./python -m venv env
env/bin/python -m pip install setuptools cd Lib/test/test_cext/
CPYTHON_TEST_EXT_NAME=test_cext CPYTHON_TEST_ABI3T=1 ../../../env/bin/python setup.py build_ext cd build/lib.linux-x86_64-cpython-315-pydebug/
mv test_cext.cpython-315d-x86_64-linux-gnu.so test_cext.abi3t.so
cd ../../../../..
PYTHONPATH=Lib/test/test_cext/build/lib.linux-x86_64-cpython-315-pydebug/ ./python -c 'import test_cext; print(test_cext.__file__)'

Output:

Lib/test/test_cext/build/lib.linux-x86_64-cpython-315-pydebug/test_cext.abi3t.so

It works as expected (I can import the C extension named .abi3t.so).

Note: I used a debug build of Python which is not good to use the stable ABI, but it was for a quick test :-)