That's a pretty good idea, deprecating & warning + providing automatic conversion utilities + using 'future' to make them errors...
But that won't happen :-(
And then import from past to remove warnings if you really want to use those constructs without warnings. Next time, feature would be removed or a depreciation warning when used with from past.
Some deprecation warnings have been enabled for a developers main source files recently. Though not in third-party libraries, which would be frustrating to the end-user.
