0.22-2021-03-26

Release dedicated to my dear son, Raffi, who turns 16 at the end of the
month. Happy birthday, dear Raffi.

• Make it easier to override the DictionaryProxy by moving it to the
  scope of the BaseDocumentViewSet.
• Add tzinfo check for better date vs datetime determination.
• Broader EmptySearch compatibility with elasticsearch_dsl.Search.
• Remove old Django code from docs.
• Allow more specific targeting for nested sort fields.
• Add more tests.