fix CI by moving exclude_lines .coveragerc setting to a new [report] §

On 21 September, all the continuous integration checks on a revision of
pull-request #920 were observed to fail with a "CoverageException:
Unrecognized option '[run] exclude_lines=' in config file
.coveragerc". One can only imagine that this was due to it trying to use
the new version 4.0 of Ned Batchelder's ever-popular coverage.py (just
released on 20 September), with which our existing .coveragerc would
seem to have not been compatible.

.coveragerc

@@ -6,6 +6,8 @@ omit =
     */site-packages/nose/*
     */pypy/*
 
+
+[report]
 exclude_lines =
     # Have to re-enable the standard pragma
     pragma: no cover