Fix datetime localization in tests and pin pytz.

Using this has been the only way I can get the tests to pass. They are passing for @reedstrm and @karenc, so I'm not sure what the problem is.

These changes make the data more accurate. That is to say, the test case posts data from a different timezone. The given iso8601 timestamp is entered into Postgresql, which will accept a timestamp from any timezone.

I don't know what the issue is. So this is a pull request that needs another pair of eyes.