Fails to detect GIL usage with Python 3.7

[benfred]

The _PyThreadState_Current symbol is no longer being used - so we won't be able to use it to figure out which thread is holding on to the GIL anymore.

It should be possible to get this from _PyRuntime.gilstate.tstate_current (if the process has symbols). Alternatively, it might be possible to call the _PyThreadState_UncheckedGet function in the target process.

[benfred]

Initial version here #104 .

Unfortunately using bindgen for pyruntimestate isn't really an option: there are OS specific member variables (like mutexes) inside this struct, and they don't have a consistent size across OS/architecture. Instead I'm computing the offset and using on a per OS/architecture/python version basis.

[benfred]

Windows implementation here - 155b467 . Will be fixed in next release