Using unittest2 backport for tests under Python 2.7

Hello all,

As per this pull request I would like to use the unittest2
backport on a case-by-case basis when running Biopython
unit tests on Python 2.7:

https://github.com/biopython/biopython/pull/961

i.e. Some tests will be skipped on Python 2.7 unless
unittest2 is installed.

We did this for a while under Python 2.6 for other things
added to the unittest library in Python 2.7 onwards. Here
the motivation is assertWarns added in Python 3.2:

https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertWarns

Assuming no objections by Friday, I'll apply this pull request,

Thanks,

Peter