Adds tox environment to test requirements conflicts

Enables developer to check requirements for conflicts instead of letting
him wondering clueless about a Zuul job failure.

Also fixes broken requirements-check job due to sphinx conflict:

ERROR: Could not find a global requirements entry to match package sphinx. If the package is already included in the global list, the name or platform markers there may not match the local settings.

Change-Id: Ia20e7907c3a60727d9402bbec1456eedc7970a51

test-requirements.txt

@@ -9,7 +9,8 @@ hacking>=1.1.0,<1.2.0 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
 docutils>=0.11 # OSI-Approved Open Source, Public Domain
 python-subunit>=1.0.0 # Apache-2.0/BSD
-sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.2; python_version=='2.7' # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2; python_version>='3.4' # BSD
 oslotest>=3.2.0 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD

tox.ini

@@ -114,3 +114,11 @@ deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
+
+[testenv:requirements]
+basepython = python3
+deps =
+  -egit+https://opendev.org/openstack/requirements#egg=openstack-requirements
+whitelist_externals = sh
+commands =
+    sh -c '{envdir}/src/openstack-requirements/playbooks/files/project-requirements-change.py --req {envdir}/src/openstack-requirements --local {toxinidir} master'