[Catalog-sig] PEP 381 timestamps

["Martin v. Löwis"]

In implementing pep381client, I noticed that the last-modified format is
underspecified. It says to use ISO 8601, but that doesn't really say
much - many different formats would be possible.

I suggest to clarify this as meaning the same format as XML-RPC uses,
i.e. "%Y%m%dT%H:%M:%S\n".

I also noticed that the naming of the file is slightly confusing: if you
don't modify any mirrored content (because the master didn't since the
last synchronisation), you are still supposed to modify last-modified
(making this file the only one that was actually modified). OTOH, the
specification makes it clear that this is the time of the last
synchronization, so there is probably no need to change anything here.

Regards,
Martin