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  bundles an outdated version of pip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 lxml
  (because the ABI tag changed) and tries to compile, which fails

tox.ini

@@ -18,10 +18,14 @@ commands =
 # Install ZEO because zodbbrowser depends on ZODB3 which depends on ZEO >=
 # 4.0.0dev which tells pip that prereleases are fine, which makes it go and
 # download ZEO 5.0.0a2, which is broken on windows, somehow.
+# Upgrade pip because otherwise we can't find lxml wheels because the ABI tag
+# format changed or something.
 deps =
     zope.testrunner
     persistent
     ZEO
+    pip
+install_command = python -m pip install -U {opts} {packages}
 
 [testenv:py]
 commands =