Minor Python 3 compatibility fixes

[carlthome]

Minor compatibility issue with pm.write() fixed.

• In Python2, range returns a list.
• In Python3, range returns an iterator.

Thus channels does not have a remove method in a Python 3 environment. This pull request fixes that by explicitly making channels into a list.

[carlthome]

Unfortunately tests fail even on the master branch in both Python 2 and 3 (without any modifications on my part) so it's hard to tell if this minor change will cause any trouble (though I seriously doubt it).

EDIT: Perhaps adding Travis to this repo would be sweet? 🎉

[carlthome]

Python 2.7 is the oldest version that has cmp_to_key. I think that should be okay, right?

[craffel]

Hey, thanks for this. There are a few other places where a comparison function is being used. Do you want to put cmp_to_key on all of them in this PR?

Python 2.7 is the oldest version that has cmp_to_key. I think that should be okay, right?

Yes, requiring 2.7 is fine.

EDIT: Perhaps adding Travis to this repo would be sweet?

This will happen sometime.

[craffel]

@carlthome now that mido is the backend, let's get this merged so that we can claim Python 3 compatibility! Looks like you need to rebase.

[carlthome]

Great news, @craffel!

Rebased and merged with current master. 👍

I'm unable to find any other places using cmp though (searching through all .py files didn't pop up anything, what am I missing?)

[craffel]

Oops, sorry, I misunderstood - thought the issue was whenever key= was used, not whenever cmp= was used. You found the only usage of cmp=. Merging!