Remove GIL for grpc_call_unref

src/python/grpcio/grpc/_cython/_cygrpc/call.pyx.pxi

@@ -85,9 +85,10 @@ cdef class Call:
     return result
 
   def __dealloc__(self):
-    if self.c_call != NULL:
-      grpc_call_unref(self.c_call)
-    grpc_shutdown_blocking()
+    with nogil:
+      if self.c_call != NULL:
+        grpc_call_unref(self.c_call)
+      grpc_shutdown_blocking()
 
   # The object *should* always be valid from Python. Used for debugging.
   @property