Make EnvironmentVarGuard picklable

Author: srinivasreddy

Lib/test/support/os_helper.py

@@ -792,6 +792,9 @@ def copy(self):
     def __enter__(self):
         return self
 
+    def __reduce__(self):
+        return (dict, (dict(self),))
+
     def __exit__(self, *ignore_exc):
         with self._lock:
             for (k, v) in self._changed.items():