Update pytz to 2018.05 (needed by babel 2.6.0)

On Ubuntu 18.04 (others not tested) installing the dev requirements
fails with the following error

babel 2.6.0 has requirement pytz>=0a, but you'll have pytz 2012d which
is incompatible.

Despite the comment in dev_requirements.txt pytz-2012d is not the most
recent version to support Python 2.6. In fact the latest release of pytz
supports Python 2.6.

dev_requirements.txt

@@ -3,7 +3,7 @@ ansible==2.6.1
 coverage==4.5.1
 Django==1.6.11  # Last version supporting 2.6.
 mock==2.0.0
-pytz==2012d  # Last 2.6-compat version.
+pytz==2018.5
 paramiko==2.3.2  # Last 2.6-compat version.
 pytest-catchlog==1.2.2
 pytest==3.1.2

tests/module_finder_test.py

@@ -353,13 +353,9 @@ class DjangoFindRelatedTest(DjangoMixin, testlib.TestCase):
             'django.utils.translation',
             'django.utils.tree',
             'django.utils.tzinfo',
-            'pkg_resources',
-            'pkg_resources.extern',
-            'pkg_resources.extern.appdirs',
-            'pkg_resources.extern.packaging',
-            'pkg_resources.extern.six',
             'pytz',
             'pytz.exceptions',
+            'pytz.lazy',
             'pytz.tzfile',
             'pytz.tzinfo',
         ])