Below, where the command tox ... is run, you will need instead to run
./.build/virtualenv/bin/tox ... when not in an activated virtual
environment.
After editing the config file, run tox config and check for lines
containing warnings like # !!! unused: ignore_base_python_conflict. This
indicates that you've put a setting in the wrong section of the tox.ini
file (in this case, in [testenv] instead of [tox]).