postinst is in python3 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 38 39 40 41 42 43 44 45 46 47 48  | #! /bin/sh
set -e
new_config_file()
{
    cat > /etc/python3/debian_config <<-EOF
	[DEFAULT]
	# how to byte-compile (comma separated: standard, optimize)
	byte-compile = standard
	EOF
}
case "$1" in
    configure)
	[ -d /etc/python3 ] || mkdir /etc/python3
	[ -f /etc/python3/debian_config ] || new_config_file
	oldv=$(echo $2 | sed 's/^\(...\).*/\1/')
	if [ "$DEBIAN_FRONTEND" != noninteractive ]; then
	    echo "running python rtupdate hooks for python3.6..."
	fi
	for hook in /usr/share/python3/runtime.d/*.rtupdate; do
	    [ -x $hook ] || continue
	    if ! $hook rtupdate python$oldv python3.6; then
	        hb=$(basename $hook .rtupdate)
	        echo >&2 "error running python rtupdate hook $hb"
	        errors=yes
	    fi
	done
	[ -z "$errors" ] || exit 4
	if [ "$DEBIAN_FRONTEND" != noninteractive ]; then
	    echo "running python post-rtupdate hooks for python3.6..."
	fi
	for hook in /usr/share/python3/runtime.d/*.rtupdate; do
	    [ -x $hook ] || continue
	    if ! $hook post-rtupdate python$oldv python3.6; then
	        hb=$(basename $hook .rtupdate)
	        echo >&2 "error running python post-rtupdate hook $hb"
	        errors=yes
	    fi
	done
	[ -z "$errors" ] || exit 5
esac
if which py3compile >/dev/null 2>&1; then
	py3compile -p python3 /usr/share/python3/
fi
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