Add default exclusion to coverage exclude_lines.

Once `exclude_lines` is set, the default exclusion is overridden, so we
can't ignore coverage on specific lines. This adds the default rule back
so that can be done again.

Author: QuLogic

.coveragerc

@@ -7,6 +7,7 @@ omit = matplotlib/_version.py
 
 [report]
 exclude_lines =
+    pragma: no cover
     raise NotImplemented
     def __str__
     def __repr__