rmtree: fix rmtree with broken symlinks on python >= 3.8

After https://github.com/python/cpython/issues/86180 func is os.open
2 jobs for fix/py38rmtree in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
passed SlapOS.Buildout.UnitTest-Master.Python2-WIP

00:04:45

passed SlapOS.Buildout.UnitTest-Master.Python3-WIP

00:14:53