Fix invalid syntax for 3.4

Signed-off-by: Pedro Lacerda <[email protected]>

Author: pslacerda

mypy/main.py

@@ -1021,7 +1021,7 @@ def parse_section(prefix: str, template: Options,
             else:
                 v = section[key]
                 if not isinstance(v, ct):  # type: ignore  # No idea
-                    raise ValueError(f"{v} is not an instance of {ct}")
+                    raise ValueError("%s not an instance of %s" % (v, ct))
         except (ValueError, KeyError) as err:
             print("%s: %s: %s" % (prefix, key, err), file=sys.stderr)
             continue