Missing psycopg2 >= 2.5 version requirement

[James Page]

The use of a cursor in a with state requires a minimum version of psycopg2 of 2.5 (see http://initd.org/psycopg/docs/cursor.html).

======================================================================
FAIL: tests.test_coordination.TestAPI.test_ensure_acquire_release_return(postgresql)
tests.test_coordination.TestAPI.test_ensure_acquire_release_return(postgresql)
----------------------------------------------------------------------
_StringException: Traceback (most recent call last):
  File "tooz/tests/__init__.py", line 29, in skip_if_not_implemented
    return func(*args, **kwargs)
  File "/«PKGBUILDDIR»/tooz/tests/test_coordination.py", line 719, in test_ensure_acquire_release_return
    self.assertTrue(lock1.acquire())
  File "tooz/drivers/pgsql.py", line 135, in acquire
    return _lock()
  File "/usr/lib/python2.7/dist-packages/retrying.py", line 49, in wrapped_f
    return Retrying(*dargs, **dkw).call(f, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/retrying.py", line 206, in call
    return attempt.get(self._wrap_exception)
  File "/usr/lib/python2.7/dist-packages/retrying.py", line 247, in get
    six.reraise(self.value[0], self.value[1], self.value[2])
  File "/usr/lib/python2.7/dist-packages/retrying.py", line 200, in call
    attempt = Attempt(fn(*args, **kwargs), attempt_number, False)
  File "tooz/drivers/pgsql.py", line 116, in _lock
    with _translating_cursor(self._conn) as cur:
  File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "tooz/drivers/pgsql.py", line 83, in _translating_cursor
    with conn.cursor() as cur:
AttributeError: __exit__

currently the dependency is un-versioned in test-requirements.txt

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/219662

[OpenStack Infra (hudson-openstack)]

Change abandoned by Julien Danjou (<email address hidden>) on branch: master
Review: https://review.openstack.org/219662