Fix buggy __slots__ example definition in _threading_local docstring

[AlexWaygood]

This appears to have existed in the docstring for 20 years without anybody noticing. It's a very minor issue, but the example __slots__ definition in this docstring actually creates 6 distinct slot descriptors on the class: 'n', 'u', 'm', 'b', 'e' and 'r'. I think this is almost certainly not what the example means to show.

We do actually point to this docstring from the docs on docs.python.org: https://docs.python.org/3/library/threading.html#threading.local

[kumaraditya303]

I think this works fine, python allows a single string as special case.

[AlexWaygood]

Ugh, what? Wow, that's counterintuitive:

>>> class Foo:
...     __slots__ = 'bar'
...     
>>> f = Foo()
>>> f.bar = 'baz'
>>> f.spam = 'what'
Traceback (most recent call last):
  File "<python-input-10>", line 1, in <module>
    f.spam = 'what'
    ^^^^^^
AttributeError: 'Foo' object has no attribute 'spam' and no __dict__ for setting new attributes
>>> list(Foo.__slots__)
['b', 'a', 'r']

Thank you!! Serves me right for not double-checking before filing a PR...

[Eclips4]

>>> list(Foo.__slots__)
['b', 'a', 'r']

Here, __slots__ is just a string, which is actually iterable, and that's why it happens :)

>>> list("meow")
['m', 'e', 'o', 'w']
>>>

Everyone makes mistakes. This happens to all of us 🙂

[AlexWaygood]

Here, __slots__ is just a string, which is actually iterable, and that's why it happens :)

Right! That's why I thought that Python would iterate over the string and create 6 unique slot descriptors on the class, since that's what it does if you assign any other iterable to __slots__, such as a tuple, list, dictionary or even collections.deque