api/cli: replace private variable access with method

src/ocrmypdf/api.py

@@ -193,7 +193,7 @@ def create_options(
     else:
         cmdline.append(os.fspath(output_file))
 
-    parser._api_mode = True
+    parser.enable_api_mode()
     options = parser.parse_args(cmdline)
     for keyword, val in deferred:
         setattr(options, keyword, val)

src/ocrmypdf/cli.py

@@ -63,6 +63,9 @@ class ArgumentParser(argparse.ArgumentParser):
         super().__init__(*args, **kwargs)
         self._api_mode = False
 
+    def enable_api_mode(self):
+        self._api_mode = True
+
     def error(self, message):
         if not self._api_mode:
             super().error(message)