fix: update doctest for pandas dtype='str' change

Pandas now uses dtype='str' instead of dtype='object' for string Index
columns. Update the expected doctest output in IDStat.name accordingly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: leotrs

xgi/stats/__init__.py

@@ -113,8 +113,9 @@ def name(self):
         ('degree', 'degree(order=3)')
         >>> H.nodes.multi([da, d3]).asdict(transpose=True).keys()
         dict_keys(['degree', 'degree(order=3)'])
-        >>> H.nodes.multi([da, d3]).aspandas().columns
-        Index(['degree', 'degree(order=3)'], dtype='object')
+        >>> cols = H.nodes.multi([da, d3]).aspandas().columns
+        >>> list(cols)
+        ['degree', 'degree(order=3)']
 
         """
         name = f"{self.func.__name__}"