>>> r.remotes.origin.fetch('+refs/pull/*:refs/heads/pull/*')
Traceback (most recent call last):
  File "", line 1, in 
  File "/usr/local/lib/python2.7/dist-packages/git/remote.py", line 613, in fetch
    res = self._get_fetch_info_from_stderr(proc, progress or RemoteProgress())
  File "/usr/local/lib/python2.7/dist-packages/git/remote.py", line 555, in _get_fetch_info_from_stderr
    for err_line, fetch_line in zip(fetch_info_lines, fetch_head_info))
  File "/usr/local/lib/python2.7/dist-packages/git/remote.py", line 555, in 
    for err_line, fetch_line in zip(fetch_info_lines, fetch_head_info))
  File "/usr/local/lib/python2.7/dist-packages/git/remote.py", line 292, in _from_line
    raise TypeError("Cannot handle reference type: %r" % ref_type_name)
TypeError: Cannot handle reference type: u"'refs/pull/1/head'"

with the command line

$ git fetch origin +refs/pull/*:refs/heads/pull/*
remote: Counting objects: 9, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 9 (delta 2), reused 9 (delta 2)
Unpacking objects: 100% (9/9), done.
From http://github.com/loic-bot/testrepo
 * [new ref]         refs/pull/1/head -> pull/1/head
 * [new ref]         refs/pull/1/merge -> pull/1/merge
 * [new ref]         refs/pull/2/head -> pull/2/head
 * [new ref]         refs/pull/2/merge -> pull/2/merge
 * [new ref]         refs/pull/3/head -> pull/3/head
 * [new ref]         refs/pull/3/merge -> pull/3/merge