util.rmtree: fix for python 3

On python3, deleting a chmoded folder causes this error:

    Traceback (most recent call last):
      File "/usr/lib/python3.7/shutil.py", line 423, in _rmtree_safe_fd
        dirfd = os.open(entry.name, os.O_RDONLY, dir_fd=topfd)
    PermissionError: [Errno 13] Permission denied: 'directory'

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "slapos.core/slapos/grid/slapgrid.py", line 607, in processSoftwareReleaseList
        software.destroy()
      File "slapos.core/slapos/grid/SlapObject.py", line 360, in destroy
        rmtree(self.software_path)
      File "slapos.core/slapos/util.py", line 267, in rmtree
        shutil.rmtree(path, onerror=chmod_retry)
      File "/usr/lib/python3.7/shutil.py", line 491, in rmtree
        _rmtree_safe_fd(fd, path, onerror)
      File "/usr/lib/python3.7/shutil.py", line 425, in _rmtree_safe_fd
        onerror(os.open, fullname, sys.exc_info())
      File "slapos.core/slapos/util.py", line 264, in chmod_retry
        func(failed_path)
    TypeError: open() missing required argument 'flags' (pos 2)

Our error handler was assuming that on python 3 the EACCESS for the
directory would happen first on an os.lstat call [1], which is true when the
top level folder is not listable, but it did not support the case where
the top level folder is listable but contain another folder that is not listable.
In that case, the first error is an os.open [2]

Tests from erp5.util (where this function came from) only covered the 1 case but
new tests in slapos.core covered the 2.

Anyway, do the "chmod folder and retry from beginning" for both 1 or 2 cases.

[1]: https://github.com/python/cpython/blob/3.6/Lib/shutil.py#L475

[2]: https://github.com/python/cpython/blob/3.6/Lib/shutil.py#L418