attrs & dataclasses false positive error with slots=True when subclassing from non-slots base class

[Hnasar]

Bug Report

With attrs.define(slots=True) and dataclass(slots=True), if the base class doesn't use __slots__ then mypy shouldn't error when assigning new attributes to the instance.

To Reproduce

import attrs
@attrs.define(slots=True)
class AttrEx(Exception):
    def __init__(self, other_exception: Exception) -> None:
        self.__traceback__ = other_exception.__traceback__  # should not error


import dataclasses
@dataclasses.dataclass(slots=True)
class DataclassEx(Exception):
    def __init__(self, other_exception: Exception) -> None:
        self.__traceback__ = other_exception.__traceback__  # should not error

class SlotsEx(Exception):
    __slots__ = ("foo",)
    def __init__(self, other_exception: Exception) -> None:
        self.__traceback__ = other_exception.__traceback__  # correctly doesn't error

https://mypy-play.net/?mypy=latest&python=3.11&gist=9abddb20dc0419557e8bc07c93e41d25

Expected Behavior

No error, since this is fine at runtime (and mypy correctly doesn't error for classes which manually define slots (and don't use attrs or dataclasses).

When inheriting from a class without slots, the dict and weakref attribute of the instances will always be accessible.
https://docs.python.org/3/reference/datamodel.html#slots

Actual Behavior

foo.py:7: error: Trying to assign name "__traceback__" that is not in "__slots__" of type "foo.AttrEx"  [misc]
foo.py:16: error: Trying to assign name "__traceback__" that is not in "__slots__" of type "foo.DataclassEx"  [misc]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.6.0+dev.d7b24514d7301f86031b7d1e2215cf8c2476bec0 (compiled: no) but this behavior is also present in 1.2 (if not earlier)

[ikonst]

Looks like the logic is to ignore __slots__ if anyone in the MRO doesn't have it:

mypy/mypy/semanal.py

Lines 4645 to 4648 in 6f650cf

	if any(p.slots is None for p in self.type.mro[1:-1]):
	# At least one type in mro (excluding `self` and `object`)
	# does not have concrete `__slots__` defined. Ignoring.
	return