Skip invalid-self tests under Python 2

Python2 doesn't allow you to invoke __init__ with an invalid `self`: it
gives you a TypeError (before the code gets to pybind):

    unbound method __init__() must be called with TestFactory1 instance
    as first argument (got TestFactory2 instance instead)

Author: jagerman

tests/conftest.py

@@ -211,6 +211,7 @@ def pytest_namespace():
         'requires_eigen_and_scipy': skipif(not have_eigen or not scipy,
                                            reason="eigen and/or scipy are not installed"),
         'unsupported_on_pypy': skipif(pypy, reason="unsupported on PyPy"),
+        'unsupported_on_py2': skipif(sys.version_info.major < 3, reason="unsupported on Python 2.x"),
         'gc_collect': gc_collect
     }
 

tests/test_factory_constructors.py

@@ -413,8 +413,11 @@ def test_reallocations(capture, msg):
     """)
 
 
+@pytest.unsupported_on_py2
 @pytest.mark.parametrize('which, bad', [(1, 1), (1, 2), (6, 1), (6, 2), (6, 3), (6, 4)])
 def test_invalid_self(which, bad, capture):
+    """Tests invocation of the pybind-registered base class with an invalid `self` argument.  You
+    can only actually do this on Python 3: Python 2 raises an exception itself if you try."""
     class NotPybindDerived(object):
         pass