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 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: I6023bc3767c5aaae90be1c5434d024e9ed76e737

doc/requirements.txt

@@ -1,5 +1,5 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-openstackdocstheme>=1.20.0 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2 # BSD
+openstackdocstheme>=2.2.1 # Apache-2.0
+sphinx>=2.0.0,!=2.1.0 # BSD

doc/source/conf.py

@@ -46,7 +46,7 @@ add_function_parentheses = True
 add_module_names = True
 
 # The name of the Pygments (syntax highlighting) style to use.
-pygments_style = 'sphinx'
+pygments_style = 'native'
 
 # -- Options for HTML output --------------------------------------------------
 
@@ -62,9 +62,9 @@ pygments_style = 'sphinx'
 html_theme = 'openstackdocs'
 
 # openstackdocstheme options
-repository_name = 'openstack/python-solumclient'
-bug_project = 'python-solumclient'
-bug_tag = ''
+openstackdocs_repo_name = 'openstack/python-solumclient'
+openstackdocs_bug_project = 'python-solumclient'
+openstackdocs_bug_tag = ''
 
 # Output file base name for HTML help builder.
 htmlhelp_basename = 'python-solumclientdoc'