[core] Acquire GIL before calling PyGILState_Release

[kevin85421]

Why are these changes needed?

For Python/CPython, the GIL should be automatically released and reacquired at every switch interval. However, Cython somehow interferes with the GIL, so when we try to call PyGILState_Release to release a thread, the following error message is displayed:

The error message shows that the current thread (which holds the GIL) is the main thread, but we are trying to release another thread (i.e. default executor). The CPython source code shows that this means that the default executor doesn't hold GIL at that time.

In this PR, we explicitly acquire GIL when we call PyGILState_Release.

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