Clarify “Sequences also support slicing” in Data Model §3.2.5 #159180

Documentation

https://docs.python.org/3.14/reference/datamodel.html#sequences

The section speaks about sequences in general, not only built-in ones(otherwise the qualifier “including built-in sequences” would be redundant). It consistently distinguishes universal properties from partial ones: negative subscripts are qualified as “Some sequences, including built-in sequences…”, and extended slicing as “Some sequences also support…”. Basic slicing, however, is stated unconditionally (“Sequences also support slicing”), although neither the glossary entry for sequence (https://docs.python.org/3.14/glossary.html) nor collections.abc.Sequence (https://docs.python.org/3.14/library/collections.abc.html#collections.abc.Sequence) requires it: a minimal class implementing only getitem/len passes isinstance(x, Sequence) but raises TypeError on slicing.

The proposed “Sequences may also support slicing (all built-in sequences do):...” wording would make the paragraph internally consistent.

Linked PRs

• gh-159180: Clarify that basic slicing is not universal for sequences #159181