bump appveyor pip version

Author: mr-c

appveyor.yml

@@ -31,6 +31,7 @@ install:
   - "python -c \"import struct; print(struct.calcsize('P') * 8)\""
 
 build_script:
+  - "%CMD_IN_ENV% pip install -U setuptools pip"
   - "%CMD_IN_ENV% pip install ."