Support Python 3

[singingwolfboy]

Fixes #142. Depends on #205 and #206 -- and I'm happy to pull some of these changes out into other, smaller pull requests if you prefer.

[mitsuhiko]

Neat! I will try to review it over the weekend.

[singingwolfboy]

@mitsuhiko: Fantastic! Now that you've merged the other PRs I made, I've rebased this branch onto the latest version of master, so there are fewer commits to look at.

As you can see from Travis, all the automated tests pass. In addition, I tried running lektor quickstart and lektor server on Python 3, and discovered a few more bugs which I also fixed, but that indicates lack of coverage in the test suite. I added two small new tests, but I wasn't sure how to test the larger stuff. Please pay special attention to the last few commits in this PRs, since those are the ones that fix bugs that do not have associated automated tests. (I'd be happy to write more automated tests, if I had someone to guide me through how to write the necessary tests.)

Once this is working properly on Python 3, I'm going to try building my own Lektor-powered site. If I do so successfully, then I volunteer to maintain Python 3 support for Lektor, since I'll be doing it anyways. :)

[singingwolfboy]

@mitsuhiko: Any update on this pull request?

[mitsuhiko]

Hi @singingwolfboy. Sorry i was so busy with other things. Any chance we could talk about this PR this week on gitter/irc?

[singingwolfboy]

@mitsuhiko has given me commit rights on the project, so that I can support Lektor on Python 3. Let me know if I've missed anything! Thanks, @mitsuhiko!

[sybrenstuvel]

When will we see a release that includes this? The last release is over a year old.

[tariquesani]

Wish there could be a fixed date. The development is stalled at the moment.

[austinbutler]

So with this merged and Lektor 3.1 released, does that mean Python 3 is supported?

[nixjdm]

@austinbutler Yes.