Fix the error message for disallowed __weakref__ slots

[maggyero]

BPO	43814
Nosy	@maggyero
PRs	gh-87980: Fix the error message for disallowed __weakref__ slots #25362

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-04-12.13:55:17.866>
labels = ['interpreter-core', 'type-feature', '3.11']
title = 'Fix the error message for disallowed __weakref__ slots'
updated_at = <Date 2022-01-15.19:22:04.426>
user = 'https://github.com/maggyero'

bugs.python.org fields:

activity = <Date 2022-01-15.19:22:04.426>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2021-04-12.13:55:17.866>
creator = 'maggyero'
dependencies = []
files = []
hgrepos = []
issue_num = 43814
keywords = ['patch']
message_count = 1.0
messages = ['390851']
nosy_count = 1.0
nosy_names = ['maggyero']
pr_nums = ['25362']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43814'
versions = ['Python 3.11']

[maggyero]

When

TypeError: __weakref__ slot disallowed: either we already got one, or __itemsize__ != 0

is raised, the second condition base->tp_itemsize != 0 (i.e. the base is a variable-length type, like int, tuple and bytes) in the error message is impossible since using a non-empty __slots__ (e.g. __slots__ = ('__weakref__',)) for a subtype of a variable-length type raises a

TypeError: nonempty __slots__ not supported for subtype of '…'

earlier in the type.__new__ implementation.