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I think this is reasonable. A patch would be welcome.
You could use inspect. When I had to do something similar I just did the call inside a try/except, caught TypeError and retried without the extra argument. See the __init__ of email.feedparser.FeedParser. I don't know which approach is better; although, with the new signature support in 3.4 perhaps inspecting the signature is better.
Another approach would be to designate a new method name for the new signature, and use hasattr to decide which to call. That's actually a bit more consistent with the way the SMTPChannel works.