Run python in unbuffered mode on Drone

The test output currently freezes while the tests are running on Drone. Then, suddenly, all the test output appears when it's done.

I assume this has got something to do with python buffering stdout, this commit disables that.

.drone.yml

@@ -25,7 +25,7 @@ pipeline:
     commands:
       - XDG_CACHE_HOME=/drone/pip-cache pip install wheel
       - XDG_CACHE_HOME=/drone/pip-cache pip install -e .[testing,docs]
-      - python runtests.py
+      - python -u runtests.py
 
 cache:
   mount: