Update appveyor.yml

Author: LaGuer

appveyor.yml

@@ -20,6 +20,7 @@ install:
   - "%CMD_IN_ENV%  python -m pip install --upgrade setuptools pip"
   - "%CMD_IN_ENV%  pip install nose coverage"
 #  - "%CMD_IN_ENV%  pip install .[test]"
+  - "%CMD_IN_ENV%  pip install pytest"
   - "%CMD_IN_ENV%  mkdir results"
   - "%CMD_IN_ENV%  cd results"
 test_script: