Follow the new PTI for document build

The Project Testing Interface [1] asks that we list requirements in
'doc/requirements.txt' and build docs by calling 'sphinx-build' directly
instead of via the 'build_sphinx' setuptool/distutils wrapper. Start
doing this.

[1] https://governance.openstack.org/tc/reference/project-testing-interface.html

Change-Id: If9342c8ea757b1735f2488db751008984fb33baf

test-requirements.txt

@@ -13,10 +13,6 @@ PyMySQL>=0.7.6 # MIT License
 python-barbicanclient>=4.5.2 # Apache-2.0
 python-ironicclient>=2.2.0 # Apache-2.0
 requests-mock>=1.1.0 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
-sphinxcontrib-actdiag>=0.8.5 # BSD
-sphinxcontrib-seqdiag>=0.8.4 # BSD
-os-api-ref>=1.4.0 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 stestr>=1.0.0 # Apache-2.0
 osprofiler>=1.4.0 # Apache-2.0
@@ -24,17 +20,10 @@ testresources>=2.0.0 # Apache-2.0/BSD
 testscenarios>=0.4 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT
 bandit>=1.1.0 # Apache-2.0
-openstackdocstheme>=1.18.1 # Apache-2.0
 gabbi>=1.35.0 # Apache-2.0
 
 # vmwareapi driver specific dependencies
 oslo.vmware>=2.17.0 # Apache-2.0
 
-# releasenotes
-reno>=2.5.0 # Apache-2.0
-
 # placement functional tests
 wsgi-intercept>=1.4.1 # MIT License
-
-# redirect tests in docs
-whereto>=0.3.0 # Apache-2.0