Replace variable annotation with type comment to fix Python 3.5 (#10690)

The wheel builds were broken. Our CI no longer uses Python 3.5.

Author: JukkaL

mypy/checkexpr.py

@@ -2777,10 +2777,8 @@ def check_boolean_op(self, e: OpExpr, context: Context) -> Type:
 
         assert e.op in ('and', 'or')  # Checked by visit_op_expr
 
-        left_map: mypy.checker.TypeMap
-        right_map: mypy.checker.TypeMap
         if e.right_always:
-            left_map, right_map = None, {}
+            left_map, right_map = None, {}  # type: mypy.checker.TypeMap, mypy.checker.TypeMap
         elif e.right_unreachable:
             left_map, right_map = {}, None
         elif e.op == 'and':