archivebot.py: make timestripper more robust

Timestripper is made more robust stripping from text parts that
are not supposed to contain timestamps.
This should reduce false positive matches.

Point 1) of observed archivebot problems on cswiki in bug 72047.

Bug: 72047

Change-Id: I715e074dedb7677bce18d20e2004c9614546e6b9
Merge "archivebot.py: make timestripper more robust"