Add filterwarnings for test_nwb_has_external_links

Fixes all warnings for 3.12

_______________________________________________________ test_nwb_has_external_links _______________________________________________________
.tox/py3/lib/python3.12/site-packages/hdmf/build/objectmapper.py:1107: in __add_datasets
    data, dtype = self.convert_dtype(spec, attr_value)
.tox/py3/lib/python3.12/site-packages/hdmf/build/objectmapper.py:264: in convert_dtype
    warnings.warn(full_warning_msg, DtypeConversionWarning)
E   hdmf.build.warnings.DtypeConversionWarning: Spec 'TimeSeries/timestamps': Value with data type int64 is being converted to data type float64 as specified.

tox.ini

@@ -48,6 +48,8 @@ markers =
     obolibrary
 filterwarnings =
     error
+    # dandi/tests/test_pynwb_utils.py::test_nwb_has_external_links
+    ignore:.*Value with data type .* is being converted:hdmf.build.warnings.DtypeConversionWarning
 
 [coverage:run]
 parallel = True