Use dtype_backend instead of is_object_dtype

Author: WillAyd

pandas/io/sql.py

@@ -45,7 +45,6 @@
 from pandas.core.dtypes.common import (
     is_dict_like,
     is_list_like,
-    is_object_dtype,
     is_string_dtype,
 )
 from pandas.core.dtypes.dtypes import (
@@ -1322,7 +1321,7 @@ def _harmonize_columns(
                 elif (
                     using_string_dtype()
                     and is_string_dtype(col_type)
-                    and is_object_dtype(self.frame[col_name])
+                    and dtype_backend == "numpy"
                 ):
                     self.frame[col_name] = df_col.astype(col_type)
                 elif dtype_backend == "numpy" and len(df_col) == df_col.count():