`drop_gil()` assumes that its caller is attached, which means that the current thread holds the GIL if and only if the GIL is enabled, and the enabled-state of the GIL won't change. This isn't true, though, because `detach_thread()` calls `_PyEval_ReleaseLock()` after detaching and `_PyThreadState_DeleteCurrent()` calls it after removing the current thread from consideration for stop-the-world requests (effectively detaching it). Fix this by remembering whether or not a thread acquired the GIL when it last attached, in `PyThreadState._status.holds_gil`, and check this in `drop_gil()` instead of `gil->enabled`. This fixes a crash in `test_multiprocessing_pool_circular_import()`, so I've reenabled it.
Miscellaneous source files for the main Python shared library