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.

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: I84ad61a25101e9bcf67908382d272b447b605fd9

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.
 
-openstackdocstheme>=1.18.1 # Apache-2.0
-reno>=2.5.0 # Apache-2.0
-sphinx>=1.8.0,!=2.1.0 # BSD
+openstackdocstheme>=2.2.1 # Apache-2.0
+reno>=3.1.0 # Apache-2.0
+sphinx>=2.0.0,!=2.1.0 # BSD

releasenotes/source/conf.py

@@ -56,9 +56,9 @@ extensions = [
 ]
 
 # openstackdocstheme options
-repository_name = 'openstack/python-blazarclient'
-bug_project = 'blazar'
-bug_tag = ''
+openstackdocs_repo_name = 'openstack/python-blazarclient'
+openstackdocs_bug_project = 'blazar'
+openstackdocs_bug_tag = ''
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
@@ -85,7 +85,7 @@ language = None
 exclude_patterns = []
 
 # The name of the Pygments (syntax highlighting) style to use.
-pygments_style = 'sphinx'
+pygments_style = 'native'
 
 
 # -- Options for HTML output -------------------------------------------------