Switch to newer openstackdocstheme version

Switch to openstackdocstheme 2.2.1 version. Using
this version will allow especially:
* Linking from HTML to PDF document
* Allow parallel building of documents
* Fix some rendering problems

Update Sphinx version as well.

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.

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.

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

Change-Id: Ie30132979d7f063de5b2a92dfdf95118541b356b

doc/source/conf.py

@@ -28,13 +28,14 @@ extensions = [
 ]
 
 # openstackdocstheme options
-repository_name = 'openstack/security-specs'
-bug_project = 'bandit'
-bug_tag = ''
+openstackdocs_repo_name = 'openstack/security-specs'
+openstackdocs_auto_name = False
+openstackdocs_bug_project = 'bandit'
+openstackdocs_bug_tag = ''
 html_theme = 'openstackdocs'
 
 # Feed configuration for yasfb
-feed_base_url = 'http://specs.openstack.org/openstack/security-specs'
+feed_base_url = 'https://specs.openstack.org/openstack/security-specs'
 feed_author = 'OpenStack Development Team'
 
 exclude_patterns = [
@@ -76,7 +77,7 @@ 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 --------------------------------------------------
 

requirements.txt

@@ -3,6 +3,6 @@
 # process, which may cause wedges in the gate later.
 pbr>=2.0.0
 doc8 # Apache-2.0
-openstackdocstheme>=1.30.0 # Apache-2.0
-sphinx>=1.8.1
+openstackdocstheme>=2.2.1 # Apache-2.0
+sphinx>=2.0.0,!=2.1.0 # BSD
 yasfb>=0.5.1