Guard pandas import in test_groupby.py (fixes CI collection failure)

pandas is an optional dependency in this repo; the module-level
`import pandas as pd` broke test collection in CI environments without
it installed. Moved to pytest.importorskip("pandas") inside the one
test that needs it, and rewrote the other new test that only used
to_pandas() for convenience to compare via the existing col() helper
instead, dropping the pandas dependency entirely there.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: FrancescAlted

tests/ctable/test_groupby.py

@@ -8,7 +8,6 @@
 from dataclasses import dataclass, make_dataclass
 
 import numpy as np
-import pandas as pd
 import pytest
 
 import blosc2
@@ -421,7 +420,8 @@ def test_groupby_engine_numpy_matches_auto():
     t = CTable(SalesRow, new_data=DATA)
     auto_result = t.group_by("city", engine="auto").sum("sales")
     numpy_result = t.group_by("city", engine="numpy").sum("sales")
-    assert auto_result.to_pandas().equals(numpy_result.to_pandas())
+    assert col(auto_result, "city") == col(numpy_result, "city")
+    np.testing.assert_array_equal(col(auto_result, "sales_sum"), col(numpy_result, "sales_sum"))
 
 
 def test_groupby_engine_jit_not_implemented():
@@ -827,6 +827,7 @@ def test_agg_duplicate_output_names_rejected():
 
 
 def test_agg_udf_matches_pandas_reference():
+    pd = pytest.importorskip("pandas")
     t = CTable(SalesRow, new_data=DATA)
     result = t.group_by("city", sort=True).agg(rng=("sales", lambda a: a.max() - a.min()))