support srearch template with extended-length path

some template file contains forward slash in the path, which is not supported by the extended path on windows.

see also:
 - https://github.com/bazelbuild/rules_python/issues/680
 - https://stackoverflow.com/questions/21194530/what-does-mean-when-prepended-to-a-file-path

sphinx/jinja2glue.py

@@ -1,6 +1,7 @@
 """Glue code for the jinja2 templating engine."""
 
 from os import path
+import os
 from pprint import pformat
 from typing import TYPE_CHECKING, Any, Callable, Dict, Iterator, List, Tuple, Union
 
@@ -117,6 +118,9 @@ class SphinxFileSystemLoader(FileSystemLoader):
     def get_source(self, environment: Environment, template: str) -> Tuple[str, str, Callable]:
         for searchpath in self.searchpath:
             filename = path.join(searchpath, template)
+            # Repalce '/' by os.sep if The searchpath is an extended-length path.
+            if filename.startswith("\\\\?\\"):
+                filename = filename.replace('/', os.sep)
             f = open_if_exists(filename)
             if f is None:
                 continue