ENH: expand versioned_branches feature to Python 3 minor version comparison (<, >, <=, >= with else)

[neutrinoceros]

Fix #488
It should cover Python 3.x branches (where x is >=0) using operators <, <=, >, >=, and assuming
that an else statement exists.

A couple remarks:

• this is my very first contribution to anything that uses ast, so I'm kind of astonished that my own tests are passing, but I'll gladly take any suggestions on how to make this better
• I ended up kinda "hacking" existing functions (namely _fix_py3_block, _fix_py3_block_else and _fix_py2_block) in the sense that I'm using them without modifications in a different fashion compared to their original intended usage. I suppose they ought to be renamed to better reflect what they actually do here (fix and if/else block and keep the top or lower half ?), but I leave this as a question to the maintainer
• I didn't cover the == operator. I can do it if you feel it's necessary, but I thought I'd ask for feedback first.

So... what do you think ? anything obvious that I'm not covering in the tests yet ?

[neutrinoceros]

Also; should I add docs too ?

[neutrinoceros]

Ah, I see that this actually only gets 3/4 cases right from the associated issue's example, so this is definitely not complete, but right now it's bedtime for me, I'll add and fix more tests tomorrow !

[neutrinoceros]

Alright, after a couple iterations (more tests and bugfixes), this is now in a state that covers every case I can currently think of correctly 🎉

[asottile]

this makes a sometimes-present variable which is a bit of a hazard

the whole patch can be very much simplified by changing (3,) => (3, 0) and then you don't need any special logic

[neutrinoceros]

this makes a sometimes-present variable which is a bit of a hazard

point taken.

the whole patch can be very much simplified by changing (3,) => (3, 0) and then you don't need any special logic

Not so sure about that: state.settings.min_version isn't guaranteed to have a second item (with --py3-plus it's just (3,)), but (3,) >= (3, 0) evaluates to False. Should I change this ?

More generally I think you're right that it can be simplified, at least it feels a lot like it, I just don't see how right now.

[asottile]

I think you can just set:

if state.settings.min_version == (3,):
    min_version = (3, 0)
else:
    min_version = state.settings.min_version

or something of the sort

[neutrinoceros]

@asottile I was able to simplify the logic in _compare_to_3, I think it doesn't get much simpler from here. I also added some docs. Now ready for your next (final ?) review :)

[neutrinoceros]

added a "simplification" that avoids repeating a check when the minor version is > 0, but I'm not sure it's worth the obfuscation. Leaving it as an isolated commit so it can be easily pruned or squashed.

[asottile]

I think this any block could be eliminated and subsumed into the _compare_to function

right now it's linear on the version being compared which could potentially be bad (?)

I'm going to merge this as is and we can always fix it later?

[neutrinoceros]

Sounds like a plan :)

[asottile]