Drop support for python 2.7
At last python 2.7 is discontinued and the whole world has moved to p3k.
Or has it ?? Mercurial core is barely ported to p3k, and as an extension, it's hard to deal with it. But we have to move forward nonetheless.
From now on, this extension will only be tested with python 3 (currently testing with 3.6 and 3.7).
Other changes are the standard bug fixes and adaptation to upstream API changes, most notably matplotlib.
Most people have a clone which they can just update, but for those oldschool, I've put the usual tarballs on the 'file' tab, up-there.