Bug 1351760

Description of problem:
All of my backups are now in fail mode because of a change in the Paramiko library which the Fedora version of obnam is not compatible with.

So this is very urgent!

Version-Release number of selected component (if applicable):
obnam-1.17-2.fc24.x86_64

Related: http://distix.obnam.org/obnam-dev/e21786a47a674589abdcee9b03f811da.html
Related: https://aur.archlinux.org/packages/obnam/

Additional info:
00h00m01s 0 files 0 B scanned: adding clientTraceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/cliapp/app.py", line 189, in _run
    self.process_args(args)
  File "/usr/lib64/python2.7/site-packages/obnamlib/app.py", line 207, in process_args
    cliapp.Application.process_args(self, args)
  File "/usr/lib/python2.7/site-packages/cliapp/app.py", line 589, in process_args
    method(args[1:])
  File "/usr/lib64/python2.7/site-packages/obnamlib/plugins/backup_plugin.py", line 163, in backup
    self.start_backup()
  File "/usr/lib64/python2.7/site-packages/obnamlib/plugins/backup_plugin.py", line 202, in start_backup
    self.prepare_repository_for_client()
  File "/usr/lib64/python2.7/site-packages/obnamlib/plugins/backup_plugin.py", line 229, in prepare_repository_for_client
    self.add_client(self.client_name)
  File "/usr/lib64/python2.7/site-packages/obnamlib/plugins/backup_plugin.py", line 349, in add_client
    self.repo.lock_client_list()
  File "/usr/lib64/python2.7/site-packages/obnamlib/fmt_6/repo_fmt_6.py", line 125, in lock_client_list
    self._raw_lock_client_list()
  File "/usr/lib64/python2.7/site-packages/obnamlib/fmt_6/repo_fmt_6.py", line 111, in _raw_lock_client_list
    self._client_list.start_changes()
  File "/usr/lib64/python2.7/site-packages/obnamlib/fmt_6/repo_tree.py", line 84, in start_changes
    self.init_forest(allow_writes=True)
  File "/usr/lib64/python2.7/site-packages/obnamlib/fmt_6/repo_tree.py", line 63, in init_forest
    allow_writes=allow_writes)
  File "/usr/lib/python2.7/site-packages/larch/forest.py", line 199, in open_forest
    check_size('key_size', key_size, BadKeySize)
  File "/usr/lib/python2.7/site-packages/larch/forest.py", line 193, in check_size
    if keyname not in ns.get_metadata_keys():
  File "/usr/lib/python2.7/site-packages/larch/nodestore_disk.py", line 167, in get_metadata_keys
    self._load_metadata()
  File "/usr/lib/python2.7/site-packages/larch/nodestore_disk.py", line 148, in _load_metadata
    data = self.journal.cat(self.metadata_name)
  File "/usr/lib/python2.7/site-packages/larch/journal.py", line 162, in cat
    return self.fs.cat(filename)
  File "/usr/lib64/python2.7/site-packages/obnamlib/repo_fs.py", line 58, in cat
    data = self.fs.cat(filename)
  File "/usr/lib64/python2.7/site-packages/obnamlib/plugins/sftp_plugin.py", line 588, in cat
    f.prefetch()
TypeError: prefetch() takes exactly 2 arguments (1 given)