* Deprecate support for Python < 3.6
This is a first step, beyond this we can:
* remove all from future imports
* remove 'six' lib as a dependency
* start using f-strings
* enjoy ourselves
* fix tox
* Rebasing
* fix
* [refactor] Remove dependency to personal fork of supercluster from mapbox visualizations
- Update dependency to reference the vanilla supercluster
- Clean up backend api call for mapbox vizzes to ensure a boolean is sent to indicate whether the viz includes custom metric for clustering
- Refactor of mapbox and its cluster overlay components to use vanilla supercluster and its recommeded way for handling clustering based on custom aggregations.
- Allow reclustering within the initial bounds on render in mapbox visualizations (stay true to old behaviors).
- Remove the median aggregation from available cluster label aggregators as there is no memory efficient way to implement this and it is unknown how often this feature is used
- Updating doc to mention the backward incompatible change re median
* Perform the check for has_custom_metric through `not None` to produce a boolean and rename the field reflect it is a boolean.
* Bump celery to 4.1.1
Docs reference `celery worker --app=superset.sql_lab:celery_app
--pool=gevent -Ofair` command which seems only to work with Celery 4.1.1
* Add UPDATING.md message
Maxime Beauchemin
Christine Chambers
Grace Guo
John Bodley