Move doc requirements into dedicated file

For the sake of homogeneity between oslo project scope,
prefer to use doc/requirements.txt

Change-Id: I10acf283752d18cb98c8ad19b8af4758bed94679

doc/requirements.txt

@@ -0,0 +1,9 @@
+# The order of packages is significant, because pip processes them in the order
+# of appearance. Changing the order has an impact on the overall integration
+# process, which may cause wedges in the gate later.
+
+# For generating sphinx documentation
+doc8>=0.6.0 # Apache-2.0
+sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7'  # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2;python_version>='3.4'  # BSD
+openstackdocstheme>=1.18.1 # Apache-2.0

test-requirements.txt

@@ -4,11 +4,7 @@
 
 hacking<0.11,>=0.10.0
 
-doc8>=0.6.0 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7'  # BSD
-sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2;python_version>='3.4'  # BSD
-openstackdocstheme>=1.18.1 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0
 testtools>=2.2.0 # MIT

tox.ini

@@ -2,10 +2,6 @@
 minversion = 2.0
 envlist = py27,py37,docs,pep8
 
-[testenv:docs]
-basepython = python3
-commands = python setup.py build_sphinx
-
 [testenv]
 install_command = pip install {opts} {packages}
 deps =
@@ -22,8 +18,6 @@ commands = flake8 {posargs}
 basepython = python2.7
 commands =
     {[testenv]commands}
-    sphinx-build -b doctest doc/source doc/build
-    doc8 --ignore-path "doc/source/history.rst" doc/source
 
 [testenv:venv]
 basepython = python3
@@ -37,8 +31,19 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 basepython = python3
 commands = python setup.py test --coverage --testr-args="{posargs}"
 
+[testenv:docs]
+basepython = python3
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
+commands =
+  doc8 --ignore-path "doc/source/history.rst" doc/source
+  sphinx-build -W doc/source doc/build
+
+
 [testenv:releasenotes]
 basepython = python3
+deps = {[testenv:docs]deps}
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:bindep]