_PyThreadState_Current_GET() is a function to get current python thread
state without asserting it is !NULL. It was added as part of d53271b9
(bigfile/virtmem: Big Virtmem lock)

We are going to adapt it to Python 3.5 (see next patch), so before doing
so move it to our compatibility place.

In the new place the name is _PyThreadState_UncheckedGet -- like such
function is named in Python 3.5 (again, see next patch).

Updates: nexedi/wendelin.core#1