[#34] add test for black import failures

Signed-off-by: Bryant Finney <bryant.finney@alumni.uah.edu>

tests/setup_updater_test.py

@@ -58,3 +58,17 @@ def test_black():  # type: () -> None
     is_black = setup_updater._get_formatting_module()
 
     assert is_black is black
+
+@pytest.mark.xfail(
+    float(python_version()[:3]) < 3.6, reason="black requires python >= 3.6"
+)
+def test_not_black(monkeypatch, dummy_import):  # type: (MonkeyPatch, Callable) -> None
+    """
+    verify `black` import failures provide an alternative formatter (or None)
+    """
+    import black
+
+    monkeypatch.setattr("builtins.__import__", dummy_import)
+    is_not_black = setup_updater._get_formatting_module()
+
+    assert is_not_black is not black