Please strip dates from EPUB files

This bug was originally reported by Axel Beckert (abe@debian.org) in Debian bug #943902:

Package: dh-strip-nondeterminism
Version: 1.6.1-1
Severity: wishlist
Control: affects -1 debian-paketmanagement-buch

For some reason, dh_strip_nondeterminism doesn't seem to strip file
timestamps from EPUB files despite strip-nondeterminism can strip out
date stamps from them (as diffoscope shows).

This also results in a few packages not being reproducibly built:
https://tests.reproducible-builds.org/debian/issues/unstable/timestamps_in_epub_issue.html