Correct line number in test output

With this and preceeding work, closing #3.

All the RST### codes now have tests, and we cover
module, method, function and class docstrings.

tests/RST203/def_list.py

@@ -3,7 +3,7 @@
 This is expected to fail validation:
 
     $ flake8 --select RST RST203/def_list.py
-    RST203/def_list.py:19:1: RST203 Definition list ends without a blank line; unexpected unindent.
+    RST203/def_list.py:21:1: RST203 Definition list ends without a blank line; unexpected unindent.
 
 In this case, the four parameters should all have their own ``:parameter``
 opening.