we need pytest 3 for some tests use `pytest.approx()`.
we no longer need patched bumpversion, as we now call `git tag` ourselves (in `python setup.py release`)
virtualenv is required by tox. Appveyor ships with virtualenv 13.x or something.
tox works fine with that.
However, virtualenv install its own embedded version of pip in the newly created environments. When we install the requirements.txt, pip complains that pip is not up-to-date. Updating virtualenv makes pip stop complaining.
Woosh.
