Fix sphinx-doc#8959: using UNIX path separator confuses Sphinx on Win…

…dows

The first element of env.relfn2path() should be a OS dependent path, not
a posix path.

CHANGES

@@ -13,6 +13,8 @@ Deprecated
 Features added
 --------------
 
+* #8959: using UNIX path separator in image directive confuses Sphinx on Windows
+
 Bugs fixed
 ----------
 

sphinx/environment/__init__.py

@@ -10,7 +10,6 @@
 
 import os
 import pickle
-import posixpath
 import warnings
 from collections import defaultdict
 from copy import copy
@@ -34,6 +33,7 @@
 from sphinx.util.docutils import LoggingReporter
 from sphinx.util.i18n import CatalogRepository, docname_to_domain
 from sphinx.util.nodes import is_translatable
+from sphinx.util.osutil import os_path
 
 if False:
     # For type annotation
@@ -351,14 +351,15 @@ def relfn2path(self, filename: str, docname: str = None) -> Tuple[str, str]:
         source dir, while relative filenames are relative to the dir of the
         containing document.
         """
+        filename = os_path(filename)
         if filename.startswith('/') or filename.startswith(os.sep):
             rel_fn = filename[1:]
         else:
             docdir = path.dirname(self.doc2path(docname or self.docname,
                                                 base=None))
             rel_fn = path.join(docdir, filename)
 
-        return (posixpath.normpath(rel_fn),
+        return (path.normpath(rel_fn),
                 path.normpath(path.join(self.srcdir, rel_fn)))
 
     @property