remove Python2 support

[a-detiste]

patsy is still needed in Debian because it's a dependency of something else

[bashtage]

patsy is more-or-less in permanent maintenace mode. I'm not sure that this patch can be accepted given the breadth of the changes needed. Why is this needed?

[a-detiste]

Hi, it's mostly about untangling the six dependencies. I've sent tens (hundreds ?) so PR, so I'm used to even if it looks a bit scary.

[a-detiste]

Removing declared six dependencies help to find undeclared six dependencies somewhere else, that's the most interesting point. So many people think that six is from the standard library and should always be there.

I think that this lib will stay forever, but limiting the risk coverage for distributions would be a nice move

[bashtage]

I don't think that removing six is a strong enough reason to accept these changes, but others who have helped continue this library should have their say.