- Add test case which should report the error code.

- Add test case where the error is suppressed due to __dict__
  in __slots__.

- Add test to detect annotation not in any base class' __slots__

- Add test against false-positive when base doesn't have __slots__
  (since there is still then an instance __dict__ thanks to base)

- Add test against false-positive when base has __slots__ with
  __dict__ entry

Ignore declare-non-slots for regression_5479.py

- This regression test is for "assigning-non-slot" so ignore
  "declare-non-slots" which also matches this example.

Implement check for "declare-non-slot" and 2 new helper methods:

- Add method `_check_declare_non_slot` to report "declare-non-slot" error.
  This method checks `node` and all bases for a valid `__slots__`, gathering
  the names in all `__slots__` found. If `node` has an annotation not in any
  `__slots__`, then "declare-non-slot" is reported.

- Add helper method `_has_valid_slots` which returns True if a valid `__slots__`
  is found on a ClassDef (re-use logic from `_check_slots`).

- Refactor `_check_redefined_slots` to split out logic for getting `__slots__`
  names into `_get_classdef_slots_names` helper.

for more information, see https://pre-commit.ci

Co-authored-by: Daniël van Noord <13665637+DanielNoord@users.noreply.github.com>

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

Avoid unnecessary inference if any ancestor has __dict__ in __slots__

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

…on-slot'

- An empty __slots__ is required if a class takes part in multiple
  inheritance, so assume this is the case if the base class has an
  empty __slots__ and abort the check.

- Abort early if __dict__ found in any __slots__.

- No need to call self._has_valid_slots in
  self._get_classdef_slots_names since we already call it at the start
  of the check.

- Move definition to below E0244 for consistency, remove "'" around %r
  as this adds extra quotes when formatted.

for more information, see https://pre-commit.ci

…annotations