Ask tox to upgrade pip

Works around the terrible mess that is the state of Python packaging:

- latest virtualenv doesn't work with Python 3.5 on Windows
  (pypa/virtualenv#796)

- virtualenv-rewrite that I use instead works with Python 3.5 but
  bundles an outdated version of pip

- old version of pip doesn't find the right binary wheels for Pillow
  (because the ABI tag changed) and tries to compile, which fails

tox.ini

@@ -5,7 +5,9 @@ envlist =
 [testenv]
 passenv =
     PIP_FIND_LINKS
+install_command = python -m pip install -U {opts} {packages}
 deps =
+    pip
     mock
 commands =
     python tests.py