Update DATETIME_FORMAT to clarify it must be UTC

[ikonst]

The DATETIME_FORMAT is

1. used as a format string for strftime but only for UTC datetimes
2. interpolated into error messages

While the %z didn't matter for (1) as it was always "+0000", for (2) it suggested that UTCDateTimeAttribute can read serialized values with different timezone offsets, which is not the case.

Fixes #1139.

[anandswaminathan]

@ikonst Just want to make sure - While the %z didn't matter for (1) as it was always "+0000", is true.

This would be a dangerous serialization change if not

[ikonst]

@anandswaminathan the serialization code is value.astimezone(timezone.utc).strftime(DATETIME_FORMAT) so I think %z would always become "+0000"