Thanks for the patch, Andrew. It looks mostly good.

I would rename setreadermode to _setreadermode (there's no reason to
make it public IMO). Also, I would not explicitly check "STARTTLS" in
the capabilities. It the server doesn't support it, it will issue an
error anyway. Some servers might support it without advertising it, who
knows?

I can take care of all that when committing, you don't need to submit a
new patch (you can of course, if you want).

> (in fact I think usenetrc should be false by default for this reason,
> but I figure that would break backwards compatibility for programs
> that rely on it being true by default, and I'm not sure what the rules
> are regarding this) 

In 3.2, the reworked nntplib already breaks compatibility. It is
reasonable to switch the default for usenetrc to False; but I prefer to
do it in a separate commit for clarity.