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.

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.

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.

See also
http://lists.openstack.org/pipermail/openstack-discuss/2020-May/014971.html

Change-Id: Ie4aec288c74b9bd8d8d117f4bc2e0282cea4db90

Make a few cleanups:
- Remove python 2.7 stanza from setup.py
- Update classifiers
- Update requirements, no need for python_version anymore

Change-Id: I6b6dfb0959973abf0c2e8325006025db2a5d85d0

Some options are now automatically configured by the version 1.20:
- project
- html_last_updated_fmt
- latex_engine
- latex_elements
- version
- release.

Change-Id: I2577ca452a014402f1377474457095176d422b42

See https://github.com/sphinx-doc/sphinx/issues/6440 for upstream details
Depend-On: https://review.opendev.org/#/c/663060/

Change-Id: I8a96eea9d3d8f6d8beae9d86f307f0edd075392c

Sphinx 2.0 no longer works on python 2.7, so we need to start capping
it there as well
depend on https://review.opendev.org/#/c/657890/

Change-Id: Ic77e0db24ab7d7f068faf3e237beefa8856a710a

We use MD5 for checksum, add this description
in cli helps.

Change-Id: I3469b0dface63f4684ad39421eee4c2a2de4d80b
Signed-off-by: Chen Hanxiao <chenhx@certusnet.com.cn>

The list of useful image properties really belongs in the Glance
Administration Guide.  The only connection they have with the
glanceclient is that you can use it to set them.  Rewrite the
property keys document to reflect this.  The dependency moves the
relevant text to the Glance Admin Guide.

Depends-on: https://review.openstack.org/565780
Change-Id: I39add6728aad42988a07d2ad97cd81dbdaf4c54a