If you simply remove the timestamp from a revision in a dump, the importer
appears to happily insert it with the current time as the timestamp. This
may also cause cancer, summon Cthulhu, etc.
In addition to pywikibot, there's the Replace Text extension[0], which
ought to be able to handle what you want to do.
[0]
https://www.mediawiki.org/wiki/Extension:Replace_Text