Introduce `get_python_state_dict()` for Python 3.12 compatibility. (#4570)

* Introduce `get_python_state_dict()`

* Conditional version bump for Python 3.12+

* Shuffle subexpressions to make the condition easier to understand (no change to logic).

* Make pybind11 ABI version 5 the minimum for Python 3.12+ (as suggested by @Lalaland)

* Add back condition for PYPY_VERSION, but keep it open for future PyPy versions.

* Fall back to simple `|| defined(PYPY_VERSION)`. `PY_VERSION_HEX` does not appear to be meaningful with PyPy.