gh-140050: keep current task state in sync between python tasks and c tasks

[kristjanvalur]

When updating the currently running task for a python impmlemented task (_PyTask), also inform the C bookkeping code in _asyncio so that query functions such as current_task() work correcty.
This allows running Python-based tasks even if the _asyncio module is available.

• Inform the _asyncio module when setting the current task, via the _enter_task(), leave_task and _swap_current_task() functions.
• use _asyncio module as primary source of truth for current_task()
• Similarly inform _asyncio module when setting current event loop and when querying it.

There are unit test cases, test_asyncio.test_events.TestPyGetEventLoop.test_get_event_loop_new_process and test_asyncio.test_futures2.PyFutureTests that started failing when this PR was partially implemented, because they use _PyTask objects. They previously just happened to work because the tasks thus created were not making use of the current_task() api.

• Issue: custom task factory and _PyTask fails to register task. #140050

[kristjanvalur]

The issue and fix have been rejected by the powers that be, so I am closing this. A fix for this issue, for those who celebrate, has been added to https://github.com/kristjanvalur/py-asynkit