Bug fix for corrupted hook state

[Archmonger]

Description

Due to some backends re-using threads across multiple requests, the hook stack could potentially become corrupted resulting in the following error: RuntimeError("Hook stack is in an invalid state")

Instead of using a ThreadLocal for the hook stack, we now use a ContextVar when ReactPy is run with a webserver. We have to use a ThreadLocal when rendering via pyodide due to lack of ContextVar support.

This PR was originally based on the work of #1204 but has deviated significantly.

As a note, this PR had to create a new pytest fixture to initialize ReactPy's hook stack. This is typically done automatically within our rendering engine, but unfortunately ContextVar values are not retained when using pytest-asyncio async fixtures.

Checklist

Please update this checklist as you complete each item:

• Tests have been developed for bug fixes or new functionality.
• The changelog has been updated, if necessary.
• Documentation has been updated, if necessary.
• GitHub Issues closed by this PR have been linked.

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[Archmonger]

@JamesHutchison I split your ContextVar changes into this PR but it doesn't appear to work within the test suite. Running the test suite results in the following error: LookupError: <ContextVar name='_hook_state' at 0x000001AC3906CF90>

These changes do seem to work fine outside the test runner though - Are there any other changes I'm supposed to migrate from #1204 to get this working?

[JamesHutchison]

The issue is likely that the tests are missing a call to create_hook_state that occurs normally during the rendering loop. I did not run the reactpy tests against my code in the marathon coding week that I wrote most of this. You could probably just add an automatic test fixture.

I do not test reactpy code automatically so this doesn't show up in Heavy Resume.