In my tox.ini, I have the following deps:
deps = -rpackaging/requirements.txt
Normally, when deps change, the virtualenv is recreated by tox. However, when the contents of requirements.txt change, tox does not notice this and the environment is reused.
A workaround would be to list the deps directly in tox.ini, but that would require duplicating the contents of the existing requirements.txt that is used by other tools.