Twisted includes some tests that operate PyDoctor, and PyDoctor depends on Twisted. So, if I put "pydoctor" under deps, and then do
tox -r -e myenv, it will install Twisted from PyPI. This is fine, but it won't install the Twisted under test (which, right now, has the same version number), meaning that I'm actually testing the released 16.3.0, not my 16.3.0+some commits.
If I subsequently run
tox -e myenv, it will correctly install the Twisted under test into the environment.