Start explicit metadata argument for marshmallow

A side effect of re-locking dependencies is having a newer marshmallow.
This deprecates having `description=` as a keyword, and puts it in a
separate metadata arg instead.

This commit is the start of my search-and-replaceing to update to the new
format.

docs/source/extensions/example_extension/06_tasks_locks.py

@@ -69,7 +69,7 @@ class TimelapseAPIView(ActionView):
             required=True, example=5, description="Number of images"
         ),
         "t_between": fields.Number(
-            missing=1, example=1, description="Time (seconds) between images"
+            load_default=1, example=1, description="Time (seconds) between images"
         ),
     }