Python virtualenv may end up containing unneeded packages

[StevenMaude]

The current prodenv/devenv simply do pip install -r requirements.txt to install dependencies.

This process will upgrade packages that were installed and need upgrading. It, however, does not remove packages that might have been legitimately previously installed, but are no longer needed.

There are cases where you might end up with unneeded packages in the virtualenv. For example:

• you switch between development branches where the requirements change
• the requirements are otherwise updated since you first install the virtualenv

Running pip-sync after installing the requirements should solve the problem, I think. It's perhaps a little bit complicated because we have separate recipes for prodenv and devenv.