chore: Type constants final and inherit enum (#27)

related from https://peps.python.org/pep-0591/

src/stmdency/constants.py

@@ -1,4 +1,8 @@
-class TOKEN:
+from enum import Enum
+from typing import Final
+
+
+class TOKEN(str, Enum):
     """Constants Token."""
 
-    EXTRACTOR_NEW_LINE = "\n\n"
+    EXTRACTOR_NEW_LINE: Final[str] = "\n\n"