test(python): Fix typo in assertion in datatype copy test (#20121)

Co-authored-by: Henry Harbeck <henry.harbeck@canstar.com.au>

py-polars/tests/unit/datatypes/test_datatype.py

@@ -7,5 +7,5 @@
 def test_datatype_copy() -> None:
     dtype = pl.Int64()
     result = copy.deepcopy(dtype)
-    assert dtype == dtype
+    assert dtype == result
     assert isinstance(result, pl.Int64)