Adds Process._counter for compat with <Py3.5

Author: ask

billiard/process.py

@@ -228,6 +228,11 @@ def sentinel(self):
         except AttributeError:
             raise ValueError("process not started")
 
+    @property
+    def _counter(self):
+        # compat for 2.7
+        return _process_counter
+
     @property
     def _children(self):
         # compat for 2.7