Assign missing codes to PyFlakes messages

Some PyFlakes messages weren't covered by unique messages, making them
impossible to select/ignore. This is now fixed.

To ensure we don't regress in the future, a test has been added that fails if
there's any uncovered messages.

tests/unit/test_pyflakes_codes.py

@@ -0,0 +1,15 @@
+"""Tests of pyflakes monkey patches."""
+
+import pyflakes
+
+from flake8.plugins import pyflakes as pyflakes_shim
+
+
+def test_all_pyflakes_messages_have_flake8_codes_assigned():
+    """Verify all PyFlakes messages have error codes assigned."""
+    messages = {
+        name
+        for name, obj in vars(pyflakes.messages).items()
+        if name[0].isupper() and obj.message
+    }
+    assert messages == set(pyflakes_shim.FLAKE8_PYFLAKES_CODES)