Fix mypy error

Author: pamelafox

app/backend/prepdocslib/textsplitter.py

@@ -2,6 +2,7 @@
 import re
 from abc import ABC
 from collections.abc import Generator
+from typing import Optional
 
 import tiktoken
 
@@ -195,7 +196,7 @@ def split_pages(self, pages: list[Page]) -> Generator[SplitPage, None, None]:
         """
         figure_regex = re.compile(r"<figure.*?</figure>", re.IGNORECASE | re.DOTALL)
 
-        previous_chunk: SplitPage | None = None
+        previous_chunk: Optional[SplitPage] = None
 
         for page in pages:
             raw = page.text or ""