Remove reno from test-requirements
Reno is only needed for building release notes. Reno has now dropped py2
support, so attempts to install all requirements for py27 jobs fail with
the latest reno release.
Reno is already correctly listed in doc/requirements.txt, so this just
removes the unnecessary entry in test-requirements.txt.
Signed-off-by: Sean McGinnis <firstname.lastname@example.org>