Fix pip call

Author: juhasch

appveyor.yml

@@ -51,9 +51,9 @@ install:
   - "python -c \"import struct; print(struct.calcsize('P') * 8)\""
   # Upgrade to the latest version of pip to avoid it displaying warnings
   # about it being out of date.
-  - 'pip install --disable-pip-version-check --user --upgrade pip'
+  - 'python -m pip install --disable-pip-version-check --user --upgrade pip'
   # install tox
-  - pip install tox
+  - python -m pip install tox
   # install of the actual project is handled by tox in tests
 test_script:
   # run tox tests