[new] py-rfc6555

Hi,

py-rfc6555 is a Python library that implements the "Happy Eyeballs"
algorithm described in RFC 6555. It's a new dependency for the latest
version of OfflineIMAP.

I was a bit unsure whether I should provide a python3 flavor. The only
consumer (OfflineIMAP) is python2 only. py-rfc6555 depends on py-selector2
which is also python2 only. The description of the later sounds like it
provides functionality backported from modern python.

OK to import?

Remi