[3.6] bpo-30395 _PyGILState_Reinit deadlock fix (GH-1734) (#1740)

head_lock could be held by another thread when fork happened. We should
reset it to avoid deadlock.
(cherry picked from commit f82c951)

Author: ambv

Misc/ACKS

@@ -552,6 +552,7 @@ Eric Groo
 Daniel Andrade Groppe
 Dag Gruneau
 Filip Gruszczyński
+Andrii Grynenko
 Grzegorz Grzywacz
 Thomas Guettler
 Yuyang Guo

Python/pystate.c

@@ -743,6 +743,10 @@ _PyGILState_Fini(void)
 void
 _PyGILState_Reinit(void)
 {
+#ifdef WITH_THREAD
+    head_mutex = NULL;
+    HEAD_INIT();
+#endif
     PyThreadState *tstate = PyGILState_GetThisThreadState();
     PyThread_delete_key(autoTLSkey);
     if ((autoTLSkey = PyThread_create_key()) == -1)