Add requirements file to manifest

Since the requirements files are read in the setup.py, they need to be
accessible for an sdist to be installed. This includes them in the
manifest.

Signed-off-by: Jeremy Cline <jeremy@jcline.org>

MANIFEST.in

@@ -5,5 +5,7 @@ include COPYING
 include COPYING.LESSER
 include createdb.py
 include README.rst
+include requirements.txt
+include dev-requirements.txt
 recursive-include alembic *
 recursive-include systemd *