Drop unittest2 support

unittest2 is a backport for Python <=3.2, so we no longer need
to support it. I suspect it has not been installed anymore after
5.0 has been released.

src/_pytest/nose.py

@@ -10,7 +10,7 @@
 
 def get_skip_exceptions():
     skip_classes = set()
-    for module_name in ("unittest", "unittest2", "nose"):
+    for module_name in ("unittest", "nose"):
         mod = sys.modules.get(module_name)
         if hasattr(mod, "SkipTest"):
             skip_classes.add(mod.SkipTest)

testing/test_unittest.py

@@ -939,9 +939,7 @@ def test_should_not_run(self):
     reprec.assertoutcome(passed=1)
 
 
-@pytest.mark.parametrize(
-    "base", ["builtins.object", "unittest.TestCase", "unittest2.TestCase"]
-)
+@pytest.mark.parametrize("base", ["builtins.object", "unittest.TestCase"])
 def test_usefixtures_marker_on_unittest(base, testdir):
     """#3498"""
     module = base.rsplit(".", 1)[0]

tox.ini

@@ -45,7 +45,6 @@ deps =
     pexpect: pexpect
     pluggymaster: git+https://github.com/pytest-dev/pluggy.git@master
     twisted: twisted
-    twisted: unittest2
     xdist: pytest-xdist>=1.13
     {env:_PYTEST_TOX_EXTRA_DEP:}
 platform = {env:_PYTEST_TOX_PLATFORM:.*}