DOC: Demo incorporating pytest into test classes
Title is self-explanatory.  Also changes CI GitHub UI photo to illustrate what most users (who are non-owners) will see (i.e. no "squash and merge").

<a href="https://github.com/pandas-dev/pandas/pull/15987#discussion_r111292547">#15987 (comment)</a>
nice picture!  @jreback All comments addressed.  Ready to merge if nothing else. thanks!