Support --disable-gil builds (PEP 703) in packaging.tags #727

The packaging.tags requires two changes to support the experimental --disable-gil builds:

1. The packaging.tags._cpython_abis() function should return "t" in the ABI for --disable-gil builds
2. The packaging.tags.cpython_tags() function should exclude "abi3" for --disable-gil builds. There is ongoing discussion on how to support features from the stable ABI, but that's more likely to take the form of an "abi4".