Create doc requirements.txt

This patch creates a new doc/requirements.txt file, moves into it the
required packages from test-requirements.txt, and makes the necessary
changes to the docs build to fix the dependencies.

Change-Id: I6cc9aadc6359fac5b985afb370bd4e33d4749b74

doc/requirements.txt

@@ -0,0 +1,7 @@
+# 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.
+
+sphinx!=1.6.6,!=1.6.7,<2.0.0;python_version=='2.7'  # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0;python_version>='3.4'  # BSD
+openstackdocstheme  # Apache-2.0

test-requirements.txt

@@ -7,6 +7,3 @@ doc8
 hacking>=1.1.0,<1.2.0 # Apache-2.0
 mock>=2.0
 requests-mock>=1.0
-sphinx!=1.6.6,!=1.6.7,<2.0.0;python_version=='2.7'  # BSD
-sphinx!=1.6.6,!=1.6.7,!=2.1.0;python_version>='3.4'  # BSD
-openstackdocstheme  # Apache-2.0

tox.ini

@@ -25,7 +25,7 @@ commands =
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
   -r{toxinidir}/requirements.txt
-  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -b html doc/source doc/build/html