appveyor: ensure test requirements are installed and up-to-date

appveyor.yml

@@ -21,9 +21,12 @@ cache:
   - '%LOCALAPPDATA%\pip\Cache'
 
 test_script:
-  - "python bootstrap.py"
-  - "python -m pip install tox"
-  - "tox -- --cov"
+  - python --version
+  - python -m pip install --disable-pip-version-check --upgrade pip setuptools wheel
+  - pip install --upgrade tox tox-venv
+  - pip freeze --all
+  - python bootstrap.py
+  - tox -- --cov
 
 after_test:
     - tox -e coverage,codecov