gh-150887: Fix Traversable abstractmethod docs

[iamrajhans]

Remove stale abstractmethod markers from importlib.resources.abc.Traversable.joinpath and truediv docs. These methods have concrete implementations.\n\nFixes gh-150887.

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #33001880 | 📁 Comparing b4f1c2c against main (a96cba5)

  🔍 Preview build  

3 files changed

± library/importlib.resources.abc.html
± using/windows.html
± whatsnew/changelog.html

[Andrew5057]

From a comment on the issue:

The implications of this in the context of a protocol are too subtle for me to feel up to proposing actual documentation text.

One option is just to not provide any doc text; the protocol inheritance semantics have more to do with protocols in general than this particular one. Alternatively, a prefix along the lines of "For classes that explicitly subclass Traversable, the default implementation..." might be sufficient?

The relevant spec is in the typing docs, though linking that might be overkill.