appveyor fix: use 'python -m pip' to upgrade pip (pypa/pip#5240)

Author: ikreymer

appveyor.yml

@@ -12,7 +12,7 @@ environment:
 
 install:        
   - "SET PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%"
-  - "pip install --disable-pip-version-check --user --upgrade pip"
+  - "python -m pip install --upgrade pip"
   - "pip install -U setuptools"
   - "pip install coverage pytest-cov"
   - "pip install cffi"