We are currently using a custom-modified unittest2, which itself was a
modified version of the standard library package unittest.

Originally, unittest2 allowed people to use later versions of unittest
in early 2.* versions of python. It also backported certain utilities
from python's 3.4 version of unittest back to python2.

We don't use python2 anymore. We don't need unittest2 anymore. This
commit removes all unittest2 and removes the custom unittest2
dependency.

As advocated by Edgar and David Roe at LMFDB#6062