feat: Modernize the async routines

[erichare]

This pull request updates the Cassandra driver to make AsyncioConnection the standard fallback event loop implementation, reflecting the removal of asyncore in Python 3.12+. It also removes legacy compatibility checks and improves documentation and testing for the asyncio reactor.

Event loop selection improvements:

• Updated the connection selection priority to use AsyncioConnection as the preferred fallback after libev, and as the default for Python 3.12+ where asyncore is no longer available. The selection order and documentation were revised accordingly. [1] [2] [3]

Asyncio reactor modernization:

• Removed outdated compatibility checks for asyncio.run_coroutine_threadsafe and references to "experimental" status, reflecting that AsyncioConnection is now a mainstream option. Updated docstrings to clarify its role and requirements. [1] [2]
• Fixed coroutine locking in _push_msg to use async with for modern asyncio syntax.
• Stopped setting the event loop globally in initialize_reactor, avoiding unnecessary side effects.

Testing improvements:

• Changed the default event loop manager in tests to asyncio and replaced asynctest.TestSelector with MagicMock for selector mocking. Added a new test for multi-timer validation with wider tolerances to account for asyncio's thread-based scheduling jitter. [1] [2] [3] [4]