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"lst[1:] if len(lst)>1 else []" is useless, lst[1:] alone does the 
same :-) So it can be simplify to:
 def _parsecap(line):
 lst = line.split()
 return lst[0], lst[1:]
You might add a real example in the capa() docstring.
About poplib_02_sock_shutdown.diff: I'm not sure that poplib is the 
right place to fix the issue... if there is really an issue. Why not 
calling shutdown directly in socket.close()? :-)
You removed self._sock, nice.