UserString subtypes parameterless `__init__`

Description

Subtypes of UserString cannot be initialized without a seq argument as the builtin str can.

Reproduction

from collections import UserString

class MyString(UserString):
    pass

>>> str()
''
>>> MyString()
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    MyString()
    ~~~~~~~~^^
TypeError: UserString.__init__() missing 1 required positional argument: 'seq'

Workaround

from collections import UserString

class MyString(UserString):
    def __init__(self, seq: object = str()) -> None:
        super().__init__(seq)

CPython versions tested on:

3.13

Operating systems tested on:

Linux

从 collections.UserString 入口开始，在 Python 3.13 上复现无参数构造示例。检查 UserString 的初始化行为，并验证没有自定义 init 的子类可以像 str() 一样被实例化，同时现有的序列初始化仍然正确。