docs: suggest creating a new virtual environment for major upgrades While not strictly necessary, it may be interesting to start from a new virtualenv rather than upgrading inside the old one. This will remove old packages once installed manually but no longer necessary. At the same time, it makes a rollback of the upgrade process more easy. Update the upgrade instructions to this effect.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.