Switch to newer openstackdocstheme and reno versions

Switch to openstackdocstheme 2.2.0 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.

Depends-On: https://review.opendev.org/728938
Change-Id: Iec4e4b3ed18e36d8ee1942e45e290ce292e7f881

doc/requirements.txt

@@ -2,6 +2,6 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 
-sphinx>=1.8.0,!=2.1.0 # BSD
-reno>=2.5.0
-openstackdocstheme>=1.20.0
+sphinx>=2.0.0,!=2.1.0 # BSD
+reno>=3.1.0 # Apache-2.0
+openstackdocstheme>=2.2.0 # Apache-2.0

doc/source/conf.py

@@ -43,6 +43,6 @@ html_theme = 'openstackdocs'
 
 # -- Options for openstackdocstheme -------------------------------------------
 
-repository_name = 'openstack/oslo.upgradecheck'
-bug_project = 'oslo.upgradecheck'
-bug_tag = ''
+openstackdocs_repo_name = 'openstack/oslo.upgradecheck'
+openstackdocs_bug_project = 'oslo.upgradecheck'
+openstackdocs_bug_tag = ''

lower-constraints.txt

@@ -4,6 +4,3 @@ enum34==1.0.4
 PrettyTable==0.7.1
 oslotest==1.5.1
 stestr==2.0.0
-Sphinx==1.8.0
-reno==2.5.0
-openstackdocstheme==1.20.0