prerm is in python3-tk 3.6.5-3.
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 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37  | #! /bin/sh
set -e
remove_bytecode()
{
    pkg=$1
    max=$(LANG=C LC_ALL=C xargs --show-limits < /dev/null 2>&1 | awk '/Maximum length/ {print int($NF / 4)}')
    dpkg -L $pkg \
	| awk -F/ 'BEGIN {OFS="/"} /\.py$/ {$NF=sprintf("__pycache__/%s.*.py[co]", substr($NF,1,length($NF)-3)); print}' \
	| xargs --max-chars="$max" echo \
	| while read files; do rm -f $files; done
    dpkg -L "$pkg" \
	| sed -ne 's,^\(/.*/\)[^/]*\.py$,\1__pycache__,p' \
	| sort -u \
	| while read dir; do \
	    if [ -d "$dir" ]; then \
		rmdir --ignore-fail-on-non-empty "$dir"; \
	    fi; \
	done
}
case "$1" in
    remove)
	remove_bytecode python3-tk
        ;;
    upgrade)
        ;;
    deconfigure)
        ;;
    failed-upgrade)
        ;;
    *)
        echo "prerm called with unknown argument \`$1'" >&2
        exit 1
    ;;
esac
 |