Refactor import of col in test_dataframe.py for improved clarity

Author: kosiew

python/tests/test_dataframe.py

@@ -33,6 +33,7 @@
     WindowFrame,
     column,
     literal,
+    col,
 )
 from datafusion import (
     functions as f,
@@ -291,8 +292,6 @@ def test_sort_unsupported(df):
 
 
 def test_aggregate_string_and_expression_equivalent(df):
-    from datafusion import col
-
     result_str = df.aggregate("a", [f.count()]).to_pydict()
     result_expr = df.aggregate(col("a"), [f.count()]).to_pydict()
     assert result_str == result_expr