GitLab

The API endpoint is called upsert

* Fixes CIM-59, CIM-60 and CIM-61

* Partial object updates are only partially supported.  Change logic
  to always serialise whole object, but exclude empty phone numbers.

* Remove unused code

* Validate models on assignment

* Simplify types.  Remove constr fields, mypy don't understand them and
  CIM validates data anyway.

* Use mixins to support optional functionality

Use CimEndpoints in CimClient constructor instead of delegating arguments