Update docs building, cleanup

Update docs building and cleanup a bit:
* Update requirements for Sphinx and openstackdocstheme for
  python 3, create doc/requirements.txt for these
* Remove unneeded doc and translation sections from setup.cfg
* Remove install_command, it's unneeded, the default is fine,
  move constraints into deps, use TOX_CONSTRAINTS instead of
  obsolete UPPER_CONSTRAINTS
* Use new variables from updated openstackdocstheme

Change-Id: I659a8736195ff621032b4fb3bd7a72fa616cf8c6
diff --git a/tox.ini b/tox.ini
index ecc7ddf..8b4a058 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,11 +6,12 @@
 
 [testenv]
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
-deps = -r{toxinidir}/test-requirements.txt
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'
 basepython = python3
 
@@ -24,9 +25,13 @@
 commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [testenv:docs]
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html -d doc/buld/doctrees doc/source doc/build/html
 
 [testenv:releasenotes]
+deps = {[testenv:docs]deps}
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html