…into multiapi_classes

* 'autorestv3' of https://github.com/Azure/autorest.python: (23 commits)
  Update package.json
  add client side validation info to mixin operations (#775)
  Update ChangeLog.md
  Update package.json
  Update ChangeLog.md
  Regen with latest core (#772)
  Update package.json
  Update ChangeLog.md
  support nullable, shift access of schema docstring etc to property / param / response level (#770)
  update testsever (#768)
  update m4 to 4.15.419 (#765)
  Revert accidental commits (#766)
  fix pylint and mypy
  if a response's schema is nullable, add Optional to typing
  add nullable and nullable items info on property and list / dict schema
  add 401 -> ClientAuthenticationError to error map (#763)
  NotImplementedError -> ValueError (#764)
  Update ChangeLog.md
  improve API version error (#752)
  fix warning for BearerTokenCredentialPolicy and no credential scopes (#758)
  ...