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.

Set openstackdocs_pdf_link to link to PDF file. Note that
the link to the published document only works on docs.openstack.org
where the PDF file is placed in the top-level html directory. The
site-preview places the PDF in a pdf directory.

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.

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.

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

Change-Id: I0740851043d2e323cf9a04d2e2579a9164393258

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
-sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
-reno>=2.5.0 # Apache-2.0
+openstackdocstheme>=2.2.1 # Apache-2.0
+sphinx>=2.0.0,!=2.1.0 # BSD
+reno>=3.1.0 # Apache-2.0

doc/source/conf.py

@@ -40,10 +40,10 @@ master_doc = 'index'
 copyright = u'2018, OpenStack Developers'
 
 # openstackdocstheme options
-repository_name = 'openstack/tacker-horizon'
-bug_project = 'tacker'
-bug_tag = ''
-html_last_updated_fmt = '%Y-%m-%d %H:%M'
+openstackdocs_repo_name = 'openstack/tacker-horizon'
+openstackdocs_pdf_link = True
+openstackdocs_bug_project = 'tacker'
+openstackdocs_bug_tag = ''
 
 # If true, '()' will be appended to :func: etc. cross-reference text.
 add_function_parentheses = True
@@ -53,7 +53,7 @@ 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 --------------------------------------------------
 

lower-constraints.txt

@@ -89,7 +89,6 @@ python-swiftclient==3.2.0
 python-tackerclient==0.8.0
 pytz==2018.3
 PyYAML==3.12
-reno==2.5.0
 requests==2.18.4
 requestsexceptions==1.4.0
 rfc3986==1.1.0
@@ -97,8 +96,6 @@ semantic-version==2.3.1
 simplejson==3.13.2
 six==1.11.0
 snowballstemmer==1.2.1
-Sphinx==1.6.5
-sphinxcontrib-websupport==1.0.1
 stevedore==1.28.0
 termcolor==1.1.0
 traceback2==1.4.0