python 3.6 compat

tests/test_simulation.py

         do_nothing_process_manager: ProcessManager,
     ):
         shared.reset()
-        called = 0
         global called
+        called = 0
 
         class MySubjectMechanism(SubjectMechanism):
             def run(self):
         do_nothing_process_manager: ProcessManager,
     ):
         shared.reset()
-        called = 0
         global called
+        called = 0
 
         class MySimulationMechanism(SimulationMechanism):
             def run(self, process_number: int = None, process_count: int = None):
         do_nothing_process_manager: ProcessManager,
     ):
         shared.reset()
-        called = 0
         global called
+        called = 0
 
         class MySubjectMechanism(SubjectMechanism):
             def run(self):
         do_nothing_process_manager: ProcessManager,
     ):
         shared.reset()
-        called = 0
         global called
+        called = 0
 
         class MySimulationMechanism(SimulationMechanism):
             def run(self, process_number: int = None, process_count: int = None):
         do_nothing_process_manager: ProcessManager,
     ):
         shared.reset()
-        called = 0
         global called
+        called = 0
 
         class MySubjectMechanism(SubjectMechanism):
             def run(self):