prerm is in libpython3.7-dbg 3.7.0~b3-1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 | #! /bin/sh
set -e
case "$1" in
    remove)
	dpkg -L libpython3.7-dbg:amd64 \
	    | awk '/\.py$/ {print $0"c\n" $0"o"}' \
	    | xargs -r rm -f >&2
        ;;
    upgrade)
        ;;
    deconfigure)
        ;;
    failed-upgrade)
        ;;
    *)
        echo "prerm called with unknown argument \`$1'" >&2
        exit 1
    ;;
esac
 |