Upgrade packaging dependencies (fix AppVeyor)

These used to come along with installing tox, but this is no longer
the case since version 3.10.
See similar fix h5py/h5py@b183c74
and tox issue tox-dev/tox#1304 .

appveyor.yml

@@ -29,6 +29,7 @@ init:
   - ps: "ls C:/Python*"
 
 install:
+  - "%PYTHON%/python.exe -m pip install --upgrade wheel pip setuptools virtualenv"
   - "%PYTHON%/python.exe -m pip install tox"
   - "%PYTHON%/python.exe -m pip install -e ."