RF: Fix for abc library Traversable class module

Fix for `abc` library `Traversable` class module: import from
`importlib.resources.abc`.

Fixes:
```
/home/runner/work/nibabel/nibabel/nibabel/testing/__init__.py:30:
 DeprecationWarning: 'importlib.abc.Traversable' is deprecated and slated for removal in Python 3.14
      from importlib.abc import Traversable
```

raised for example at:
https://github.com/nipy/nibabel/actions/runs/9637811213/job/26577586721#step:7:157

Documentation:
https://docs.python.org/3/library/importlib.resources.abc.html#importlib.resources.abc.Traversable

Author: jhlegarreta

nibabel/testing/__init__.py

@@ -27,8 +27,8 @@
 from .np_features import memmap_after_ufunc
 
 try:
-    from importlib.abc import Traversable
     from importlib.resources import as_file, files
+    from importlib.resources.abc import Traversable
 except ImportError:  # PY38
     from importlib_resources import as_file, files
     from importlib_resources.abc import Traversable