The nose test framework started as a clone of pytest when pytest was at version 0.8. Its tagline is "nose extends unittest to make testing easier" (found at http://nose.readthedocs.org/en/latest/). All good things must come to an end, however; nose is currently in maintenance mode and it has been for several years.
As unittest only works on versions of Python 2.7 and greater, unittest2 was created as a backport for earlier versions. In addition to unittest's functionality, it also includes an improved API and better assertions. In effect, unittest2 is not only a backport but also a revision of unittest.
The changes to unittest were important enough that the successor to nose—imaginatively named nose2—was based on the unittest2 plugins branch. From the home page:
nose2 is the next generation of nicer testing for Python, based on the plugins branch of unittest2. nose2 aims to improve on nose by:
Providing a better plugin API
Being easier for users to configure
Simplifying internal interfaces...