Pin pytz to < 2017.3, to avoid DateTime test failure.

Failure in test testLegacyTimezones (DateTime.tests.testDateTime.DateTimeTests)
Traceback (most recent call last):
  File "/opt/python/2.7.13/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "/home/travis/build/zopefoundation/Zope/eggs/DateTime-2.12.8-py2.7.egg/DateTime/tests/testDateTime.py", line 667, in testLegacyTimezones
    self.failUnless(name.lower() in cache._zidx, 'legacy timezone  %s cannot be looked up' % name)
  File "/opt/python/2.7.13/lib/python2.7/unittest/case.py", line 612, in deprecated_func
    return original_func(*args, **kwargs)
  File "/opt/python/2.7.13/lib/python2.7/unittest/case.py", line 422, in assertTrue
    raise self.failureException(msg)
AssertionError: legacy timezone  Canada/East-Saskatchewan cannot be looked up

version_ranges.cfg

@@ -99,3 +99,6 @@ mechanize = < 0.3.dev
 python-gettext = < 1.3.dev
 zope.testbrowser = < 3.12.dev
 
+# time zones, 2017.3 gives a test error in DateTime:
+# AssertionError: legacy timezone  Canada/East-Saskatchewan cannot be looked up
+pytz = < 2017.3