Use pyversion variable to change bytes checking for python 3 less than 3.5 (#2191)

Author: buckbaskin

mypy/checkexpr.py

@@ -966,8 +966,15 @@ def visit_op_expr(self, e: OpExpr) -> Type:
             # Expressions of form [...] * e get special type inference.
             return self.check_list_multiply(e)
         if e.op == '%':
-            if isinstance(e.left, (StrExpr, BytesExpr, UnicodeExpr)):
-                return self.strfrm_checker.check_str_interpolation(e.left, e.right)
+            pyversion = self.chk.options.python_version
+            if pyversion[0] == 3:
+                if isinstance(e.left, BytesExpr) and pyversion[1] >= 5:
+                    return self.strfrm_checker.check_str_interpolation(e.left, e.right)
+                if isinstance(e.left, StrExpr):
+                    return self.strfrm_checker.check_str_interpolation(e.left, e.right)
+            elif pyversion[0] <= 2:
+                if isinstance(e.left, (StrExpr, BytesExpr, UnicodeExpr)):
+                    return self.strfrm_checker.check_str_interpolation(e.left, e.right)
         left_type = self.accept(e.left)
 
         if e.op in nodes.op_methods:

mypy/checkstrformat.py

@@ -57,7 +57,6 @@ def __init__(self,
 
     # TODO: In Python 3, the bytes formatting has a more restricted set of options
     # compared to string formatting.
-    # TODO: Bytes formatting in Python 3 is only supported in 3.5 and up.
     def check_str_interpolation(self,
                                 str: Union[StrExpr, BytesExpr, UnicodeExpr],
                                 replacements: Node) -> Type: