Fixes serializer tests failing locally in Ginkgo

[johnbaldwin]

• The version 2.4.2 dateutil.parser.parse method returns a datetime object in local time, causing the tests to fail when compared against datetime in UTC
• This commit addresses the issue by replacing the timezone with UTC
• This commit includes a helper function, as_datetime_utc in tests/test_serializers.py
• Read the docstring for this helper function

[johnbaldwin]

@bryanlandia I updated per your request

[bryanlandia]

Thanks @johnbaldwin Looks like as_datetime_utc could probably be used here too https://github.com/appsembler/figures/blob/95ac4ac2153166387a7142f12a92908efa8c0647/tests/test_filters.py#L207