manifest: Fix absolute path checking on windows for Python 3.13

Python 3.13 changed the behavior for os.path.isabs
https://docs.python.org/3/library/os.path.html#os.path.isabs

Prevent absolute paths that start with a '/'.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>

src/west/manifest.py

@@ -2421,7 +2421,9 @@ def _load_project(self, pd: Dict, url_bases: Dict[str, str],
         # symbolic links.
         ret_norm = os.path.normpath(ret.path)
 
-        if os.path.isabs(ret_norm):
+        # Python 3.13 os.path.isabs on Windows returns False for a single
+        # leading (back)slash.
+        if ret_norm.startswith('/') or os.path.isabs(ret_norm):
             self._malformed(f'project "{ret.name}" has absolute path '
                             f'{ret.path}; this must be relative to the '
                             f'workspace topdir' +