Build using Python 3
Upstream switched the shebangs to Python 3 in a later release without actually making any substantiative changes, so we should be fine running them under Python 3 as well: https://github.com/catchorg/Catch2/commit/695a356f.
Closes: #942960
I diffed the 1.12.1-1 amd64 deb and my build with this change and had no differences caused by this patch.