Add more accurate type annotations for dirs.

Though not documented, it's possible for `dirs` to contain
`pathlib.Path`.

`django.template.loaders.app_directories.Loader` is an example for this:
https://github.com/django/django/blob/03eec9ff6cc78e7c1bcf88bb76ecd11f0d433c72/django/template/loaders/app_directories.py
https://github.com/django/django/blob/03eec9ff6cc78e7c1bcf88bb76ecd11f0d433c72/django/template/utils.py#L97-L111

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

django-stubs/template/loaders/filesystem.pyi

@@ -1,13 +1,14 @@
-from typing import Iterator, List, Optional
+from pathlib import Path
+from typing import Iterator, List, Optional, Union
 
 from django.template.base import Origin
 from django.template.engine import Engine
 
 from .base import Loader as BaseLoader
 
 class Loader(BaseLoader):
-    dirs: Optional[List[str]] = ...
-    def __init__(self, engine: Engine, dirs: Optional[List[str]] = ...) -> None: ...
-    def get_dirs(self) -> List[str]: ...
+    dirs: Optional[List[Union[str, Path]]] = ...
+    def __init__(self, engine: Engine, dirs: Optional[List[Union[str, Path]]] = ...) -> None: ...
+    def get_dirs(self) -> List[Union[str, Path]]: ...
     def get_contents(self, origin: Origin) -> str: ...
     def get_template_sources(self, template_name: str) -> Iterator[Origin]: ...