Change Summary

Rename get_validators to __get_validators__ on custom classes to be more like dataclasses (ref #334), also because it's a more sensible name.

Raise a depreciation warning on use of the old name.

We'll remove get_validators usage and warning in future.

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes
• No performance deterioration (if applicable)
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • if you're not a regular contributer please include your github username @whatever