drop support for Python 3.8

[alex]

Fixes #4581.

Drops support for Python 3.8, which reached end-of-life in October 2024. Follows the same approach as the Python 3.7 drop in #5912.

• Bumps the minimum supported CPython version to 3.9 (pyo3-build-config, pyo3-ffi build script and package metadata).
• Removes the abi3-py38 feature; abi3-py39 is now the lowest stable-ABI target.
• Removes Py_3_9 cfg gates (now always true) and deletes code that only existed for 3.8, including:
  • the backports.zoneinfo fallback in PyTzInfo::timezone (and the corresponding CI install step)
  • the pre-3.9 buffer-protocol / tp_dictoffset / tp_weaklistoffset fixups in create_type_object.rs
  • the assert_dict_supported / assert_weakref_supported compile-time assertions and their abi3_dict / abi3_weakref UI tests, since dict and weakref are always available from abi3-py39
  • dead FFI items (PyCFunction_New/NewEx externs, Py*_ClearFreeList, PyImport_Cleanup, tp_print, pre-3.9 PyObject_IS_GC / PyObject_CheckBuffer fallbacks, etc.)
• Updates CI matrices, noxfile (abi3 forward-compat tests now use abi3-py39), requires-python in the various pyproject.tomls, and documentation.

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