fix [#26252] Py error when installing addon through UI

Py3.1 for windows doesn't have os.path.samefile(), has been added in 3.2.

release/scripts/ui/space_userpref.py

@@ -1089,7 +1089,8 @@ class WM_OT_addon_install(bpy.types.Operator):
         addon_path = ""
         pyfile_dir = os.path.dirname(pyfile)
         for addon_path in addon_utils.paths():
-            if os.path.samefile(pyfile_dir, addon_path):
+            # if os.path.samefile(pyfile_dir, addon_path):  # Py3.2 only!, upgrade soon!
+            if (hasattr(os.path, "samefile") and os.path.samefile(pyfile_dir, addon_path)) or pyfile_dir == addon_path:
                 self.report({'ERROR'}, "Source file is in the addon search path: %r" % addon_path)
                 return {'CANCELLED'}
         del addon_path