Tox fails in py27 because of requirements.txt difference

[spoltier]

Problem

Running pip-compile in a Python 3.6 virtual environment will produce a requirements.txt file with dependency versions that are not available under Python 2.7.
Switching manually to a 2.7 venv and running pip-compile -U fixes the tests in 2.7, but breaks them for 3.6.

Desired solution

Ideally, we would find a way to regenerate requirements.txt within each virtual environment in tox, before test commands are run.

Steps taken

I tried moving regeneration and installation to the commands in tox.ini,but that didn't generate a different requirements file, which means that tox calls the wrong pip-compile (e.g. whichever is in the enclosing environment, not the one that should be called given the environment the test commands are run in).

[spoltier]

Note that tox-dev/tox#167 does not solve the problem, as commands_pre are not run prior to installing dependencies.

[lambiase]

@spoltier : Python 2.7 will not be maintained anymore next year (https://pythonclock.org) - do you really need to test against it?

[spoltier]

@lambiase I'm aware. Just documenting the issue with the project as is, tox.ini does have py27 configured. We can also decide to remove Python 2.7 as a target, especially if this type of problem only happens going from 2 to 3.

[spoltier]

removed py27 in tox.ini