Before this change, if a UUID instance got received as value in the
Schema, it was breaking the unmarshal because UUID instances can't be
used as values to instantiate other UUIDs.
`strict_rfc3339.rfc3339_to_timestamp()` is tz aware
`datetime.datetime.fromtimestamp` is not
this causes the test `test_string_format_datetime` to fail on
system that are not on utc