Build broken by datetime changes in PyYAML 5.3

[dtkav]

Description

The following change to PyYAML ( yaml/pyyaml#163 ) altered the behavior of loading yaml files.
As of PyYAML 5.3, datetime strings like 2020-01-01T00:00:00+00:00 will be loaded as timezone-aware datetimes.

Expected behaviour

This test currently rely on PyYAML loading TZ-naive datetimes:
https://github.com/zalando/connexion/blob/master/tests/test_flask_encoder.py#L66

PyYAML < 5.3 will return 2000-01-23T04:56:07.000008Z
PyYAML >= 5.3 will return 2000-01-23T04:56:07.000008+00:00

We should update the tests to allow the changes in PyYAML 5.3

Actual behaviour

We currently depend on the timezone-naive yaml loader from PyYAML < 5.3

Steps to reproduce

Run Travis CI on master

Potential Fix

#1125

[dtkav]

closed by #1125