Switch to newer openstackdocstheme and reno versions

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 these requirements from lower-constraints, they are not needed
during installation.

openstackdocstheme renames some variables, so follow the renames. 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: I26d5cf722635d12c3cdfde6ec25b0d3e16377b2f

doc/requirements.txt

@@ -2,9 +2,9 @@
 # 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
-openstackdocstheme>=1.20.0 # Apache-2.0
-reno>=2.5.0 # Apache-2.0
+sphinx>=2.0.0,!=2.1.0 # BSD
+openstackdocstheme>=2.2.1 # Apache-2.0
+reno>=3.1.0 # Apache-2.0
 
 # Optional dependencies that are needed to build docs
 fixtures>=3.0.0 # Apache-2.0/BSD

doc/source/conf.py

@@ -28,9 +28,9 @@ extensions = [
 ]
 
 # openstackdocstheme options
-repository_name = 'openstack/oslo.log'
-bug_project = 'oslo.log'
-bug_tag = ''
+openstackdocs_repo_name = 'openstack/oslo.log'
+openstackdocs_bug_project = 'oslo.log'
+openstackdocs_bug_tag = ''
 
 # The master toctree document.
 master_doc = 'index'

lower-constraints.txt

@@ -27,7 +27,6 @@ msgpack==0.5.6
 netaddr==0.7.19
 netifaces==0.10.6
 openstack-requirements==1.2.0
-openstackdocstheme==1.20.0
 os-client-config==1.29.0
 oslo.config==5.2.0
 oslo.context==2.20.0
@@ -48,14 +47,11 @@ python-mimeparse==1.6.0
 python-subunit==1.2.0
 pytz==2018.3
 PyYAML==3.12
-reno==2.7.0
 requests==2.18.4
 requestsexceptions==1.4.0
 rfc3986==1.1.0
 smmap2==2.0.3
 snowballstemmer==1.2.1
-Sphinx==1.8.0
-sphinxcontrib-websupport==1.0.1
 stestr==2.0.0
 stevedore==1.28.0
 systemd-python==234