Enhancements to QThreadExecutor

[kristjanvalur]

This PR enhances the conformity of QThreadExecutor with the Executor protocol.

• shutdown() is now thread-safe and there is no race between shutdown() and submit().
• shutdown() can be called more than once. This is how the standard Executors work, and is congruent with close() and similar methods.
• map() is supported.
• cancel_futures argument to shutdown() is supported.

In addition, a context manager, closing() is added, which does a shutdown(wait=False) by default. This is useful in code like

with QThreadExecutor(1).closing() as executor:
    await asyncio.get_running_loop().run_in_executor(executor, my_blocking_function)

Because otherwise a blocking wait for thread completion is performed at the end of the context. It is sufficient to shut down the executor and allow the threads to wake up and stop in their own time, and not force a wait there which can last several milliseconds.

[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_qthreadexec.py					117-135, 153, 185-187
Project Total

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