Explore support for abi3t ABI tag

[ngoldbaum]

PEP 803 proposes a new abi3t ABI tag.

I've added preliminary support for abi3t in packaging and setuptools and @mgorny added preliminary support in meson-python.

The builder package will need to be updated, presumably by adding new abi3t keyword arguments for get_soabi and get_python_library and by updating the wheel tag logic.

I think there would also need to be new abi3t example packages to go along side the abi3 packages.

We've been working on a test suite for packaging libraries and bindings generators, see https://github.com/Quansight-Labs/stable-abi-testing.

[henryiii]

Should be pretty easy. The main interface for users now is py-api, which currently supports values like cp39 to target the stable ABI above a certain value. I think we could add two features: the ability to set multiple values, separated by dots, and a new setting cp315t. Like cp39, this would be ignored on the "other" interpreter, and if the value is too high, producing a regular wheel. CMake's FindPython might also need updating. We could also define a new CMake cached variable if needed.

We could ship this sooner than the PEP / 3.15 gated by experimental=True.