autotypes: stop using the deprecated tuple interface of intersphinx inventory items (RemovedInSphinx10Warning) #556

Noticed while working on #554 / #555. Not a failure today, but it becomes one with Sphinx 10.

Symptom

Builds and tests that resolve a reference through documenteer.ext.autotypes.xrefs's intersphinx fallback emit, on Sphinx 8.2 and later:

src/documenteer/ext/autotypes/xrefs.py:879: RemovedInSphinx10Warning: The tuple interface for _InventoryItem objects is deprecated. Please access the `project_name`, `project_version`, `uri`, and `displayname` attributes directly.
  return make(cast("str", entries[name][2]))
src/documenteer/ext/autotypes/xrefs.py:914: RemovedInSphinx10Warning: The tuple interface for _InventoryItem objects is deprecated. Please access the `project_name`, `project_version`, `uri`, and `displayname` attributes directly.
  uri = cast("str", entries[name][2])

The test suite shows it in tests/ext/autotypes_test.py::test_autotypes_inherited_docstring_links, and a plain sphinx-build of tests/roots/test-autotypes-inherited prints it too.

Where

Both uses are in src/documenteer/ext/autotypes/xrefs.py (line numbers as of #555):

• _resolve_intersphinx, line 879: entries[name][2]
• _intersphinx_terminal_index, line 914: entries[name][2]

Index 2 is the item's URI. A grep of src/ for ][2] finds no other uses.

Constraint on the fix

pyproject.toml declares Sphinx>=8,<10. The _InventoryItem class with a .uri attribute exists in Sphinx 8.2.3 and 9.x (checked in the nox environments), but I did not check Sphinx 8.0 and 8.1, where inventory entries may still be plain tuples. So the fix is one of:

• read .uri when the item has it and fall back to [2] otherwise, or
• raise the Sphinx floor to a release where .uri is always present, and use .uri directly.

Either way the cast("str", ...) around the access can probably go once .uri is used.

To do

• Check whether Sphinx 8.0 / 8.1 inventory entries have a .uri attribute, and pick one of the two approaches above
• Replace both entries[name][2] accesses
• Confirm the warning is gone on the Sphinx 9 and Sphinx dev test sessions (the dev session is where removal will first show up as an error)
• Before widening the dependency to Sphinx 10, grep for other RemovedInSphinx10Warning sources in the test output