Comment 1 for bug 1529143

Looking at the code a bit more:

* we actually _do_ override our TestSkipped with SkipTest, if we can find it
* we currently prefer `unittest2.SkipTest` to `unittest.SkipTest`, which I think is wrong
* Twisted confuses this by mutating the module name of stdlib SkipTest to be `twisted.trial.unittest`

Thus,

* deleting TestSkipped should wait until after we drop Python 2.6 support
* we should prefer unittest.SkipTest to unittest2.SkipTest. That is, should only use unittest2.SkipTest if the stdlib SkipTest is unavailable