postrm is in libpython3.4-minimal 3.4.3-1ubuntu1~14.04.7.
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
if [ "$1" = "purge" ]; then
    pc=$(dpkg-query -f '${db:Status-Abbrev} ${binary:Package}\n' -W libpython3.4-minimal \
	| grep -v '^.n' | wc -l)
    if [ "$pc" -lt 1 ]; then
	find /usr/lib/python3.4 -depth -type d -name __pycache__ \
	    | xargs -r rm -rf
	rm -f /etc/python3.4/sitecustomize.py
	rm -rf /etc/python3.4/__pycache__
	if [ -d /etc/python3.4 ]; then
	    rmdir --ignore-fail-on-non-empty /etc/python3.4
	fi
    fi
fi
exit 0
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