coverage: Update pattern for __str__ for new typing syntax.

The new Python 3 typing syntax means the def lines for __str__ can
look a bit different.

tools/coveragerc

@@ -10,7 +10,7 @@ exclude_lines =
     # Don't require coverage for test suite AssertionError -- they're usually for clarity
     raise AssertionError
     # Don't require coverage for __str__ statements just used for printing
-    def __str__[(]self[)]:
+    def __str__[(]self[)] -> .*:
 
 [run]
 omit =