[Debian] certbot のエラー pkg_resources.DistributionNotFound

$sudo certbot
Traceback (most recent call last):
  File "/usr/bin/certbot", line 6, in 
    from pkg_resources import load_entry_point
  File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 3019, in 
    @_call_aside
  File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 3003, in _call_aside
    f(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 3032, in _initialize_master_working_set
    working_set = WorkingSet._build_master()
  File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 655, in _build_master
    ws.require(__requires__)
  File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 963, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 849, in resolve
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'ndg-httpsclient' distribution was not found and is required by requests

Let’s Encryptのcertbotを実行したときにこんな感じのエラーが出る時、
python-pkg-resourcesを再インストールすることで（多分）解決します。

$ sudo apt-get remove --purge python-pkg-resources
$ sudo apt-get install python-pkg-resources