fix(handler): fix an invalid type hint (#46)

This fixes a small type annotation error in `handler.py` -- the
annotation had `ExceptionHandler or None` instead of `ExceptionHandler |
None`. This isn't detected by Python's own syntax checking because of
deferred annotations, but Pyright flags it.

Author: woodruffw

src/fastapi_problem/handler.py

@@ -204,7 +204,7 @@ def new_exception_handler(  # noqa: PLR0913
 
 def add_exception_handler(  # noqa: PLR0913
     app: FastAPI,
-    eh: ExceptionHandler or None = None,
+    eh: ExceptionHandler | None = None,
     *,
     logger: logging.Logger | None = None,
     cors: CorsConfiguration | None = None,