gh-146636: PEP 803: add Py_TARGET_ABI3T and .abi3t.so extension (GH-146637)

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

This is part of the implementation for PEP-803.
Detailed documentation to come later.

Co-authored-by: Nathan Goldbaum <[email protected]>