Before I started testtools I was switching regularly between bzr, Launchpad and Twisted and was sick of one excellent testing innovation being available in one and not the other. I tried submitting some of the improvements to Python. They were either ignored or rejected.
Also, as the maintainer of Twisted's testing framework, Trial, I had been burned several times when the standard library would change its API in backwards compatible ways, so I wanted something that would encapsulate all of that once and for all.
So, to provide a temporary ground between big, upstream projects that are serious about testing and Python's standard library, I made the project that was to become testtools. (It was originally called pyunit3k. Sorry.).
Its constraints were that it should work with Python 2.4, 2.5, 2.6, 2.7 and 3.1 (Bazaar, Launchpad & Twisted all cared about Python 2.4 at the time), that any code in it should be allowed to go into Python without special permission from contributors, and that it wasn't to have any frivolous innovation, patches should be of code proven to be useful elsewhere.
Then eventually Michael Foord started maintaining Python's standard unittest. While we haven't always seen eye-to-eye on the details, it's definitely been a huge improvement, and Michael deserves props for doing a job that I was unwilling to do. Releasing unittest2 was a great step, and I think a good model of how any standard library maintenance should be done.
Since then, we've lightened the restraint on innovation in testtools, and the last release was our last release to support Python 2.4 and 2.5. The people who use it really love it, we've had plenty of contributors, I use it on heaps of projects, and it certainly makes my job much, much easier. Launchpad and Bazaar use it, but Twisted probably won't ever, so that's 66% success according to my original motivation.