Switch to openstackdocstheme 2.2.1 and reno 3.1.0 versions. Using
these versions will allow especially:
* Linking from HTML to PDF document
* Allow parallel building of documents
* Fix some rendering problems
Update Sphinx version as well.
Change pygments_style to 'native' since old theme version always
used 'native' and the theme now respects the setting and using 'sphinx'
can lead to some strange rendering.
Remove docs requirements from lower-constraints, they are not
needed during install or test but only for docs building.
openstackdocstheme renames some variables, so follow the
renames before the next release removes them. A couple of variables
are also not needed anymore, remove them.
See also
http://lists.openstack.org/pipermail/openstack-discuss/2020-May/014971.html
Change-Id: Ic5ff4981e5ed09885fd2aaad7508d58d2b2cccc7
b77f7b9fe7