Deprecate `PythonAbi::from_build_env` in favor of a new `PythonAbi::from_stable_abi` constructor, which takes the stable ABI kind and minimum version explicitly instead of reading the `abi3`/`abi3t` cargo features from the environment (feature-based selection is now internal to PyO3's build scripts).

`InterpreterConfig::from_sysconfigdata` no longer selects a target ABI based on the content of abi3 or abi3t cargo features and will select a version-specific ABI instead.
