Try enforcing methods overwrite at import time.

Currently Spawners need to overwrite start, stop, poll. When this is not
done, it will fail at runtime.

This replicate this check at class definition time, meaning that
potential errors will be caught way earlier. It also have not runtime
cost as the check is a class definition time (ie often import time).

This takes only effect on Python 3.6+ which introduce __init_subclass__,
we could do it with metaclasses, but that's might be too complicated.

If one want to create a class the avoid these restriction they can
overwrite __init_subclass__ and not call the super() method.

jupyterhub/tests/test_spawner.py

@@ -18,7 +18,7 @@ from tornado import gen
 from ..user import User
 from ..objects import Hub
 from .. import spawner as spawnermod
-from ..spawner import LocalProcessSpawner
+from ..spawner import LocalProcessSpawner, Spawner
 from .. import orm
 from .utils import async_requests
 
@@ -246,3 +246,24 @@ def test_shell_cmd(db, tmpdir, request):
     r.raise_for_status()
     env = r.json()
     assert env['TESTVAR'] == 'foo'
+
+
+def test_inherit_overwrite():
+    """On 3.6+ we check things are overwritten at import time
+    """
+    if sys.version_info >= (3,6):
+        with pytest.raises(NotImplementedError):
+            class S(Spawner):
+                pass
+
+
+def test_inherit_ok():
+    class S(Spawner):
+        def start():
+            pass
+
+        def stop():
+            pass
+
+        def poll():
+            pass