tox.ini & Setup.cfg cleanups

Address suggestions from https://review.opendev.org/#/c/700679:
* remove install_command from tox.ini, this is the default
* Use sphinx-build as modern way to run sphinx, remove setup.cfg
  settings for build-sphinx
* Add python-requires to setup.cfg to make Python 3.6 requirement
  explicit

Change-Id: Ib93f6f3822c7f9d9013f095268700e3a19d252fb

setup.cfg

@@ -6,6 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/mox3/latest/
+python-requires = >=3.6
 classifiers =
     Environment :: OpenStack
     Programming Language :: Python
@@ -25,12 +26,3 @@ packages =
 [global]
 setup-hooks =
     pbr.hooks.setup_hook
-
-[build_sphinx]
-source-dir = doc/source
-build-dir = doc/build
-all_files = 1
-warning-is-error = 1
-
-[upload_sphinx]
-upload-dir = doc/build/html

tox.ini

@@ -5,7 +5,6 @@ ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
-install_command = pip install {opts} {packages}
 deps =
        -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
@@ -13,7 +12,9 @@ deps =
 commands = stestr run --slowest {posargs}
 
 [testenv:docs]
-commands = python setup.py build_sphinx
+commands =
+  rm -rf doc/build/html doc/build/doctrees
+  sphinx-build -W --keep-going -b html -d doc/build/doctrees doc/source doc/build/html
 
 [testenv:pep8]
 deps =