Update docs building

Update docs building:
* Switch to sphinx-build
* Update requirements for Sphinx and openstackdocstheme for
  python 3, create doc/requirements.txt file.
* Remove unneeded doc and translation sections from setup.cfg
* Remove install_command, it's unneeded, the default is fine.
* Use TOX_CONSTRAINTS instead of obsolete UPPER_CONSTRAINTS

Change-Id: I6ff4427014fc967a6f7b28fc6cbe5a0f4eca601c
diff --git a/tox.ini b/tox.ini
index 0f5ad24..28ac9e7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -7,11 +7,10 @@
 [testenv]
 basepython = python3
 usedevelop = True
-install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
-deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
 
@@ -32,9 +31,12 @@
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-commands = python setup.py build_sphinx
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
+commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
+deps = {[testenv:docs]deps}
 whitelist_externals = make
 commands = sphinx-build -b latex doc/source doc/build/pdf
            make -C doc/build/pdf