fix failure on test_alter_int_pk_to_int_unique

Author: michiya

sql_server/pyodbc/schema.py

@@ -277,7 +277,9 @@ def _alter_field(self, model, old_field, new_field, old_type, new_type,
             for sql, params in post_actions:
                 self.execute(sql, params)
         # Added a unique?
-        if not old_field.unique and new_field.unique:
+        if (not old_field.unique and new_field.unique) or (
+            old_field.primary_key and not new_field.primary_key and new_field.unique
+        ):
             self.execute(self._create_unique_sql(model, [new_field.column]))
         # Added an index?
         # constraint will no longer be used in lieu of an index. The following