Refactor: cts: Drop cts-lab dependency on python3-dateutil

It doesn't look like we care about the journal timestamps for any reason
other than deciding whether one message came after another. For that, we
can compare seconds since epoch, rather than parsing and comparing ISO
8601 timestamps.

I wrote the code that gets timestamps in ISO 8601 format in commit
c2fd798c, and I don't know why I did that.

This also gets rid of the only internal use of the iso8601 command line
tool. pcs is the only other known user. It has a deprecated function
that runs the iso8601 tool.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>