Move python threading stuff to own file in bf_python_ext

This way blender player can easily use BPY_thread_save/restore.
Not so much important for master branch, but crucial to solve
linking issues in threaded depsgraph branch.

source/blender/python/intern/bpy_interface.c

@@ -408,31 +408,6 @@ void BPY_python_reset(bContext *C)
 	BPY_modules_load_user(C);
 }
 
-/* wrapper functions related to global interpreter lock. these functions
- * are slightly different from the original Python API, don't throw
- * SIGABRT even if the thread state is NULL. */
-
-/* analogue of PyEval_SaveThread() */
-BPy_ThreadStatePtr BPY_thread_save(void)
-{
-	PyThreadState *tstate = PyThreadState_Swap(NULL);
-	/* note: tstate can be NULL when quitting Blender */
-
-	if (tstate && PyEval_ThreadsInitialized()) {
-		PyEval_ReleaseLock();
-	}
-
-	return (BPy_ThreadStatePtr)tstate;
-}
-
-/* analogue of PyEval_RestoreThread() */
-void BPY_thread_restore(BPy_ThreadStatePtr tstate)
-{
-	if (tstate) {
-		PyEval_RestoreThread((PyThreadState *)tstate);
-	}
-}
-
 static void python_script_error_jump_text(struct Text *text)
 {
 	int lineno;