test: Add py2py3-venv to combines two virtual environments into one

Add script that can be used to combine python2 venv and python3 venv
into one environment where python2 refers to python interpreter with
access to eggs from py2 environment and python3 refers to python
interpreter with access to eggs from py3 environment.

In the next patch we will use this script to combine py2/py3 venvs into
one environment to run `python2 gen_testdata.py` and `python3
gen_testdata.py`. Though the script should be useful on its own to do
any combined py2/py3 work or testing.

zodbtools/test/py2py3-venv

+#!/bin/bash -e
+
+# Copyright (C) 2024  Nexedi SA and Contributors.
+#                     Kirill Smelkov <kirr@nexedi.com>
+#
+# This program is free software: you can Use, Study, Modify and Redistribute
+# it under the terms of the GNU General Public License version 3, or (at your
+# option) any later version, as published by the Free Software Foundation.
+#
+# You can also Link and Combine this program with other software covered by
+# the terms of any of the Free Software licenses or any of the Open Source
+# Initiative approved licenses and Convey the resulting work. Corresponding
+# source of such a combination shall include the source code for all other
+# software used.
+#
+# This program is distributed WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+#
+# See COPYING file for full licensing terms.
+# See https://www.nexedi.com/licensing for rationale and options.
+
+# py2py3-venv combines two virtual environments into one.
+#
+# The first virtual environment should be created with python2, and the second with python3.
+# In the destination environment:
+#
+# - program python2 becomes python interpreter with access to eggs from py2 environment.
+# - program python3 becomes python interpreter with access to eggs from py3 environment.
+#
+# Similarly programs pip2 and pip3 refer to pip in py2 and py3 environment correspondingly.
+#
+# Default programs python and pip without version suffix point to python2 and pip2 correspondingly.
+
+if test $# != 3; then
+    echo "Usage: py2py3-venv <py2-venv> <py3-venv> <py2py3-venv>" 1>&2
+    exit 1
+fi
+
+py2_venv="$1"
+py3_venv="$2"
+py2py3_venv="$3"
+
+# die <msg>
+die() {
+    echo "$@" 1>&2; exit 1
+}
+
+test -e "$py2_venv"    || die "E: $py2_venv does not exist"
+test -e "$py3_venv"    || die "E: $py3_venv does not exist"
+test -e "$py2py3_venv" && die "E: $py2py3_venv already exists"
+
+test -e "$py2_venv/bin/python2" || die "E: $py2_venv is not a python2 venv"
+test -e "$py3_venv/bin/python3" || die "E: $py3_venv is not a python3 venv"
+
+py2_venv=$(cd "$py2_venv" && pwd)   # abspath
+py3_venv=$(cd "$py3_venv" && pwd)   # abspath
+
+mkdir "$py2py3_venv"
+
+# python2/python3 do not correctly activate their environments when symlinked
+cat > "$py2py3_venv/python2" <<EOF
+#!/bin/sh
+exec "$py2_venv/bin/python2" "\$@"
+EOF
+cat > "$py2py3_venv/python3" <<EOF
+#!/bin/sh
+exec "$py3_venv/bin/python3" "\$@"
+EOF
+chmod a+x "$py2py3_venv/python2"
+chmod a+x "$py2py3_venv/python3"
+
+# for pip it is ok to symlink as pip itself is a program referring to abspath of underlying python
+ln -s -T "$py2_venv/bin/pip2"    "$py2py3_venv/pip2"
+ln -s -T "$py3_venv/bin/pip3"    "$py2py3_venv/pip3"
+
+# default python / pip
+ln -sT python2 "$py2py3_venv/python"
+ln -sT pip2    "$py2py3_venv/pip"
+
+# env.sh
+cat > "$py2py3_venv/env.sh" <<EOF
+X=\${1:-\${BASH_SOURCE[0]}}       # path to original env.sh is explicitly passed
+X=\$(cd \$(dirname \$X) && pwd)    # when there is other env.sh wrapping us
+
+export PATH="\$X:\$PATH"
+export PS1="(\$(basename \$X)) \$PS1"
+EOF