devel/py-unittest2: Revive deleted port

This port is still of value. The built-in unittest in Python 2.7 is
missing features provided by unittest2 which backports from Python 3.x.

Example code which produces a failure with 2.7's built-in unittest:

import unittest2
import unittest

class ExampleUT(unittest.TestCase):

def test_unittest(self):
    for i in [1, 2, 3]:
        with self.subTest(i=i):
            self.assertEqual(i, i)

class ExampleUT2(unittest2.TestCase):

def test_unittest2(self):
    for i in [1, 2, 3]:
        with self.subTest(i=i):
            self.assertEqual(i, i)

if name == 'main':

unittest2.main()