add tox env for running requirements-check

Set up a tox environment for running project-requirements-change.py,
the script used in the requirements-check job.

Change-Id: I5336222949acfd947f2ef54ed120e3632b28ccb7
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

README.rst

@@ -71,6 +71,16 @@ requirements is produced. If the inputs are lower-constraints.txt, the
 output includes the highest version of each package referenced in the
 files.
 
+check-requirements
+------------------
+
+Run the validation checks from the ``requirements-check`` job locally
+using the ``requirements-check`` tox environment.
+
+Run::
+
+    $ tox -e requirements-check -- /path/to/repo/to/test
+
 Proposing changes
 =================
 

tox.ini

@@ -95,6 +95,12 @@ basepython = python3.5
 [testenv:py36]
 basepython = python3.6
 
+[testenv:requirements-check]
+basepython = python3
+deps = -r{toxinidir}/requirements.txt
+commands =
+  {toxinidir}/playbooks/files/project-requirements-change.py --local {posargs}
+
 [testenv:babel]
 # Use the local upper-constraints.txt file
 deps = Babel