Add a Traversable.read_text() errors parameter

[kurtmckee]

This PR introduces the following changes, in three commits:

1. Demonstrate importlib.abc.Traversable.read_text() incompatible with importlib.resources._functional.read_text() usage (Python 3.13) cpython#127012.

   This adds an in-memory finder, loader, and traversable implementation,
   which allows the Traversable protocol and concrete methods to be tested.

   This additional infrastructure demonstrates the original issue,
   but also highlights that the Traversable.joinpath() concrete method
   raises TraversalError which is not getting caught in several places.

2. Catch TraversalError, raised by Traversable.joinpath().

   Exercising the Traversable protocol's concrete methods
   has highlighted that .joinpath() raises TraversalError,
   which needs to be caught in several places.

   This is primarily resolved within the test suite,
   but implicates the is_resource() function as well.

3. Resolve a TypeError lurking in the read_text() functional API.

   importlib_resources.read_text() calls the Traversable.read_text()
   concrete method with an errors argument that doesn't exist in the
   method signature, resulting in an TypeError.

   This is resolved by adding an errors parameter to
   Traversable.read_text().

Fixes python/cpython#127012

[jaraco]

I've force-pushed the three commits after rebasing on main. Now tests are failing because I've re-enabled mypy checks (that were previously disabled).

________________________________________________________ importlib_resources/tests/util.py ________________________________________________________
279: error: Argument 1 to "reduce" has incompatible type "Callable[[dict[str, object], str], object]"; expected "Callable[[dict[str, object], str], dict[str, object]]"  [arg-type]
279: error: Incompatible return value type (got "object", expected "dict[str, object]")  [return-value]
284: error: Argument 1 to "reduce" has incompatible type "Callable[[dict[str, object], str], object]"; expected "Callable[[dict[str, object], str], dict[str, object]]"  [arg-type]
284: error: Incompatible return value type (got "object", expected "dict[str, object]")  [return-value]

[jaraco]

In the latest commit, I consolidated the repeated pattern of resolving a fullname from the fixtures, and this incidentally masks the type ambiguity.

[jaraco]

Thanks again for this PR. The work you did here was exemplary - it was clear, understandable, complete, and directly addressed the issue. Bravo.