Removed workaround for appveyor builds

- has been fixed in pytest-dev/py#170
- closes #330

Author: mrbean-bremen

appveyor.yml

@@ -6,7 +6,6 @@ environment:
     - PYTHON: "C:\\Python36-x64"
 
 install:
-  - "%PYTHON%\\python.exe -m pip install py==1.4.34"
   - "%PYTHON%\\python.exe -m pip install -r requirements.txt"
   - "%PYTHON%\\python.exe -m pip install ."