Fix the order of subclasses in doctest

The order of __subclasses__() is not fixed in the Python API. So the doctest of itersubclasses() may fail if this changes, like in the [i386 build on Python 3.4 on Ubuntu Trusty](https://launchpadlibrarian.net/162632981/buildlog_ubuntu-trusty-i386.python-astropy_0.3-5~u2_FAILEDTOBUILD.txt.gz). For fixing, we sort the output first.