Cleanups to ensure GIL-safety of Py<T> and PyObject methods

[davidhewitt]

Closes #949

This introduces Python arguments to Py<T> and PyObject methods which really should be requiring the GIL.

Doing this higlighted a couple of other places across the codebase which could be tweaked:

• PyTypeObject::type_object now returns &PyType instead of Py<PyType> - should be faster as now no reference count increase needed.
• PyTuple::slice and PyTuple::split_from now return &PyTuple.

[kngwyu]

Thank you for this hard work!
I have a minor concern about naming but other than that LGTM 👍

[davidhewitt]

Updated to use EnsureGIL struct instead of enum 👍

[kngwyu]

LGTM, thanks.