This creates a dedicated doc/requirements.txt for doc-only
dependencies like sphinx packages so we can decouple those
from the test-requirements.txt.
As a result, the missing oslo.serialization runtime dependency
is added (I used the same version that python-novaclient uses).
Due to change If558f184c959e4b63b56dec3ca1571d1034cfe5c in the
openstack/requirements repo, in order to pass the
requirements-check job we have to specify both the py27 and py3
versions for sphinx.
Change-Id: I8bc3330a456f8bb76a17c4277a094a64fe9be449
573ca6ed59