Fix reporting of UndefinedLocal pyflakes error

tests/unit/test_pyflakes_codes.py

@@ -1,4 +1,5 @@
 """Tests of pyflakes monkey patches."""
+import ast
 
 import pyflakes
 
@@ -13,3 +14,20 @@ def test_all_pyflakes_messages_have_flake8_codes_assigned():
         if name[0].isupper() and obj.message
     }
     assert messages == set(pyflakes_shim.FLAKE8_PYFLAKES_CODES)
+
+
+def test_undefined_local_code():
+    """In pyflakes 2.1.0 this code's string formatting was changed."""
+    src = '''\
+import sys
+
+def f():
+    sys = sys
+'''
+    tree = ast.parse(src)
+    checker = pyflakes_shim.FlakesChecker(tree, (), 't.py')
+    message_texts = [s for _, _, s, _ in checker.run()]
+    assert message_texts == [
+        "F823 local variable 'sys' defined in enclosing scope on line 1 referenced before assignment",  # noqa: E501
+        "F841 local variable 'sys' is assigned to but never used",
+    ]