gh-96364: Fix text signatures of __getitem__ for list and dict

[sobolevn]

After this change:

Python 3.12.0a0 (heads/main-dirty:e860e521ec, Aug 28 2022, 21:36:18) [Clang 11.0.0 (clang-1100.0.33.16)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> help(list.__getitem__)
Help on method_descriptor:

__getitem__(self, key, /)
    Return self[key].

>>> help(dict.__getitem__)
Help on method_descriptor:

__getitem__(self, key, /)
    Return self[key].

• Issue: Improve signature of __getitem__ for list and dict #96364

[sobolevn]

Thank you!