Create separate doc requirements

... instead of adding requirements only required by doc build in
test requirements.

Notes about old pip's behavior are also removed.

Change-Id: If40ad5216e3025ea6b21eba7e392f9fc9c9c3f27

doc/requirements.txt

@@ -0,0 +1,3 @@
+sphinx>=2.0.0,!=2.1.0 # BSD
+reno>=3.1.0 # Apache-2.0
+openstackdocstheme>=2.2.1 # Apache-2.0

requirements.txt

@@ -1,7 +1,3 @@
-# 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.
-
 oslo.config>=3.9.0 # Apache-2.0
 oslo.messaging>=5.2.0 # Apache-2.0
 oslo.utils>=4.7.0 # Apache-2.0

test-requirements.txt

@@ -1,12 +1,5 @@
-# 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.
-
 coverage>=3.6 # Apache-2.0
 python-subunit>=0.0.18 # Apache-2.0/BSD
-sphinx>=2.0.0,!=2.1.0 # BSD
-openstackdocstheme>=2.2.1 # Apache-2.0
 oslotest>=1.10.0 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0
-reno>=3.1.0 # Apache-2.0
 betamax>=0.7.0 # Apache-2.0

tox.ini

@@ -8,11 +8,11 @@ ignore_basepython_conflict = True
 basepython = python3
 usedevelop = True
 setenv =
-   VIRTUAL_ENV={envdir}
+    VIRTUAL_ENV={envdir}
 deps =
-       -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
-       -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -r{toxinidir}/requirements.txt
+    -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
@@ -32,6 +32,10 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
+deps =
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -r{toxinidir}/requirements.txt
+    -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html -d doc/build/doctrees doc/source doc/build/html
 
 [testenv:debug]