Fix jinja_loader typehint (#5389)

CHANGES.rst

@@ -3,6 +3,8 @@ Version 3.0.2
 
 Unreleased
 
+-   Correct type for ``jinja_loader`` property. :issue:`5388`
+
 
 Version 3.0.1
 -------------

src/flask/sansio/scaffold.py

@@ -9,6 +9,7 @@
 from functools import update_wrapper
 
 import click
+from jinja2 import BaseLoader
 from jinja2 import FileSystemLoader
 from werkzeug.exceptions import default_exceptions
 from werkzeug.exceptions import HTTPException
@@ -272,7 +273,7 @@ def static_url_path(self, value: str | None) -> None:
         self._static_url_path = value
 
     @cached_property
-    def jinja_loader(self) -> FileSystemLoader | None:
+    def jinja_loader(self) -> BaseLoader | None:
         """The Jinja loader for this object's templates. By default this
         is a class :class:`jinja2.loaders.FileSystemLoader` to
         :attr:`template_folder` if it is set.