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.