Python 3.11+: Add `__notes__` to `error_already_set::what()` output. (#4678)

* First version adding `__notes__` to `error_already_set::what()` output.

* Fix trivial oversight (missing adjustment in existing test).

* Minor enhancements of new code.

* Re-enable `cmake --target cpptest -j 2`

* Revert "Re-enable `cmake --target cpptest -j 2`"

This reverts commit 60816285e9e7b95b7d33a60805888b80b2bec641.

* Add general comment explaining why the `error_fetch_and_normalize` code is so unusual.