Update appveyor config

Author: sdispater

appveyor.yml

@@ -22,6 +22,10 @@ install:
   - pip install virtualenv
   - virtualenv --python="C:/Python36-x64\\python.exe" "venv-poetry"
 
+  # Create current python virtualenv
+  - "virtualenv --python=%PYTHON%\\python.exe venv"
+  - venv\\Scripts\\activate.bat
+
   # Install dependencies
   - pip install codecov
   - venv-poetry\\Scripts\\pip.exe install poetry --pre