Fixes Build: Allow new datetime behavior in PyYAML 5.3

[dtkav]

Fixes Master, which is broken due to PyYAML change in 5.3
yaml/pyyaml#163

Changes proposed in this pull request:

• update test to not expect Zulu datetime ending, as PyYAML 5.3 parses the date as TZ aware

[dtkav]

@hjacobs master is broken due to a change in PyYAML ( yaml/pyyaml#163 ).
This change seems reasonable to me (loading a spec with a datetime+tz will make a tz-aware datetime).
Alternatively we could pin 5.2, or 5.3 and choose one behavior. This PR accepts both.

[coveralls]

Coverage increased (+0.02%) to 96.284% when pulling f575f0e on dtkav:pyyaml_5_3_datetime_update into f55cb1c on zalando:master.

[hjacobs]

👍