This partially reverts commit 5a3a94eb51.
It should be re-introduced once the rest of comments are addressed.
Session changes are kept for the future, even if unused for now,
as there may be people who have already upgraded their session.
This behaviour is deprecated and will be removed in future versions
of Python. Technically, it could be considered a bug (invalid usage
causing different behaviour from the expected one), and in practice
it should not break much code (because .get_event_loop() would likely
be the same event loop anyway).
