test: skip generic operation test that uses pandas if pandas is not i…

…nstalled

ibis/expr/operations/tests/test_generic.py

@@ -94,7 +94,7 @@ def test_coerced_to_value(typehint, value, expected):
 
 
 def test_coerced_to_interval_value():
-    import pandas as pd
+    pd = pytest.importorskip("pandas")
 
     expected = ops.Literal(1, dt.Interval("s"))
     pat = Pattern.from_typehint(ops.Value[dt.Interval])