dataclass(slots=True) fails if method has an empty `__class__` cell

Bug report

Bug description:

On Python 3.14, this code:

def maker():
    if False:
        __class__ = 42

    def method(self):
        return __class__
    return method

from dataclasses import dataclass

@dataclass(slots=True)
class X:
    a: int

    meth = maker()

Fails with ValueError: Cell is empty, because some dataclasses machinery for updating the __class__ cell doesn't reckon with the possibility that the cell may be empty.

CPython versions tested on:

3.14, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-148947: dataclasses: fix error on empty __class__ cell  #148948
• [3.14] gh-148947: dataclasses: fix error on empty __class__ cell (GH-148948) #148995