ENH: Make linear_iterator return an iterator

Make `linear_iterator` return an iterator: `range` is an iterable, so
call `iter` on `range` in order to return an iterator.

Fixes:
```
src/nifreeze/utils.py:56: error:
 Incompatible return value type (got "range", expected "Iterator[int]")  [return-value]
src/nifreeze/utils.py:56: note:
 "range" is missing following "Iterator" protocol member:
src/nifreeze/utils.py:56: note:
     __next__
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12437972140/job/34728973936#step:8:35

src/nifreeze/utils.py

@@ -54,7 +54,7 @@ def linear_iterator(size: int | None = None, **kwargs) -> Iterator[int]:
     if size is None:
         raise TypeError("Cannot build iterator without size")
 
-    return range(size)
+    return iter(range(size))
 
 
 def random_iterator(size: int | None = None, **kwargs) -> Iterator[int]: