Maybe the solution is to leave OSError catching after the conditional
(probably with some additional comments):

   if not (exist_ok and path.isdir(name)):
      try:
          mkdir(name, mode)
      except OSError as e:
          if not exist_ok or e.errno != errno.EEXIST or not path.isdir(name):
              raise

This should solve the problem. It also gives more or less guarantee
that errno will be EEXIST, while the current code will also raise an exception
on EROFS (read-only FS), ENOSPC (no space left) in addition to EACCES on Windows
(and possibly some other values on various systems - who knows?).