github workflow: disable unavailable python3.4

https://github.com/fphammerle/free-disk/pull/6/checks?check_run_id=367653111#step:3:9

.github/workflows/python.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       matrix:
         python-version:
-        - 3.4
+        # 3.4 not available
         - 3.5
         - 3.6
         - 3.7