REF: remove unused code.
ENH: add iteration protocol to DecisionsSet.
ENH: add support for Rule.required_package.

Those different rules really should be different classes as
reason_details is of a different type depending on the rule_type. For
now, we use this hack to follow composer's design.
BUG: handle MATCH_NONE in Pool.matches.
ENH: DefaultPolicy can now handle multiple providers for a requirement.
ENH: core solver now mostly implemented.
BUG: a few py3 fixes.
ENH: consider update/ugprade/remove jobs handled.
BUG: fix python3 compatibility.