Derive outcomes.exit.Exception from SystemExit instead of KeyboardInterrupt

This is required for properly getting out of pdb, where
KeyboardInterrupt is caught in py36 at least.

Ref: https://github.com/pytest-dev/pytest/issues/1865#issuecomment-242599949

testing/test_runner.py

@@ -553,7 +553,7 @@ def test_outcomeexception_passes_except_Exception():
 def test_pytest_exit():
     with pytest.raises(pytest.exit.Exception) as excinfo:
         pytest.exit("hello")
-    assert excinfo.errisinstance(KeyboardInterrupt)
+    assert excinfo.errisinstance(pytest.exit.Exception)
 
 
 def test_pytest_fail():