Seatsio

More flexible channels API:
* moved channel management to `client.events.channels`
* added the ability to add, remove and edit a single channel
* added the ability to add and remove objects to and from a channel
* renamed `channels.update()`to `channels.replace()` for clarity

Deleted legacy fields from usage reports:

`num_first_bookings_or_selections` (use `num_used_objects` instead)
`num_ga_selections_without_booking`
`num_non_ga_selections_without_booking`

Dropped Python 2.7 support. 3.7 or newer is now required.
`Event.categories` contains `Category` domain objects instead of plain objects.

ability to add & remove a chart category

* Call that creates events in season returns events instead of season
* When retrieving an event, it now contains the list of categories that are available on its chart

Added status_change.is_present_on_chart and status_change.not_present_on_chart_reason