bpo-30395 _PyGILState_Reinit deadlock fix (#1734)

head_lock could be held by another thread when fork happened. We should
reset it to avoid deadlock.

Author: fried

Misc/ACKS

@@ -556,6 +556,7 @@ Eric Groo
 Daniel Andrade Groppe
 Dag Gruneau
 Filip Gruszczyński
+Andrii Grynenko
 Grzegorz Grzywacz
 Thomas Guettler
 Yuyang Guo

Python/pystate.c

@@ -744,6 +744,10 @@ _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)