Switch to sphinx-build and cleanup

1.Sync sphinx dependency with global requirements. It caps python 2
  since sphinx 2.0 no longer supports Python 2.7.

2. Switch to using modern sphinx-build.
3. Remove oslotest, testtools packages from test-requirements.txt,
   their not used.

Change-Id: Ia25e4a06ca2ec4e0018d5763e0946ded704b521b

setup.cfg

@@ -6,6 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = http://www.openstack.org/
+python-requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -13,10 +14,9 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
-    Programming Language :: Python :: 2
-    Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =

test-requirements.txt

@@ -5,11 +5,8 @@
 hacking>=0.12.0,<0.13 # Apache-2.0
 
 coverage>=4.0,!=4.4 # Apache-2.0
-python-subunit>=0.0.18 # Apache-2.0/BSD
-sphinx>=1.6.2 # BSD
-oslotest>=1.10.0 # Apache-2.0
+sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.6'  # BSD
 stestr>=1.0.0 # Apache-2.0
-testtools>=1.4.0 # MIT
 openstackdocstheme>=1.11.0  # Apache-2.0
 # releasenotes
 reno>=1.8.0 # Apache-2.0

tox.ini

@@ -6,7 +6,6 @@ skipsdist = True
 [testenv]
 basepython = python3
 usedevelop = True
-install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
@@ -36,7 +35,7 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-commands = python setup.py build_sphinx
+commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
 commands =