1.4: Fix running tests with Python 3.14

1.4: Fix running tests with Python 3.14

Backported to SQLAlchemy 1.4 an improvement to the test suite with regards
to how asyncio related tests are run, now using the newer Python 3.11
``asyncio.Runner`` or a backported equivalent, rather than relying on the
previous implementation based on ``asyncio.get_event_loop()``.  This allows
the SQLAlchemy 1.4 codebase to run on Python 3.14 which has removed this
method.  Pull request courtesy Nils Philippsen.

Cleaned up some old cruft in tox.ini and added py314 markers.

backported adef933f8d12938 from 2.0 to update test_memusage to use
the "fork" context

Fixes: #12668
Closes: #12666
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12666
Pull-request-sha: 6f0f4d6cca4cfcb60bd4ed09323eae5e5bd04958

Change-Id: I1f9682f945836e1e347cf1693eb30ff43ebe6e30