RF: Use numpy.lib.scimath to demonstrate warning context manager

Use `numpy.lib.scimath` instead of deprecated `numpy.core.fromnumeric`
in `clear_and_catch_warnings` context manager doctests.

Take advantage of the commit to add an actual case that would raise a
warning.

Fixes:
```
nibabel/testing/__init__.py::nibabel.testing.clear_and_catch_warnings
  <doctest nibabel.testing.clear_and_catch_warnings[1]>:1:
 DeprecationWarning: numpy.core is deprecated and has been renamed to
 numpy._core. The numpy._core namespace contains private NumPy internals
 and its use is discouraged, as NumPy internals can change without
 warning in any release. In practice, most real-world usage of
 numpy.core is to access functionality in the public NumPy API. If that
 is the case, use the public NumPy API. If not, you are using NumPy
 internals. If you would still like to access an internal attribute, use
 numpy._core.fromnumeric.
```

raised for example at:
https://github.com/nipy/nibabel/actions/runs/9692730430/job/26746686623#step:7:195

Author: jhlegarreta

nibabel/testing/__init__.py

@@ -150,9 +150,10 @@ class clear_and_catch_warnings(warnings.catch_warnings):
     Examples
     --------
     >>> import warnings
-    >>> with clear_and_catch_warnings(modules=[np.core.fromnumeric]):
+    >>> with clear_and_catch_warnings(modules=[np.lib.scimath]):
     ...     warnings.simplefilter('always')
-    ...     # do something that raises a warning in np.core.fromnumeric
+    ...     # do something that raises a warning in np.lib.scimath
+    ...     _ = np.arccos(90)
     """
 
     class_modules = ()