Convert "ensure_future" to "create_task"

[kristjanvalur]

asyncio.ensure_future is an anacronism. Since python version 3.7, create_task() is preferred over ensure_future.

Moreover, error handling is more natural (and traceback friendly) by using wrapper methods instead of
future completion callbacks.

• We add some unit tests for the asyncSlot() exception case.
• We trigger error handling for asyncClose()
• We ignore CancelledError for asyncClose()
• We add unit tests for asyncClose()

[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/qasync
__init__.py
_windows.py
tests
test_qeventloop.py					900
Project Total

This report was generated by python-coverage-comment-action

[hosaka]

@kristjanvalur thank you for contributing!

[kristjanvalur]

I should add that the previous implementation of asyncClose had a problem: Any uncaught exceptions in the close slot would be silently ignored, since the future's result() was never evaluated. I came across this in production recently so it is worth getting this pr released at some point.