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

src/_pytest/main.py

@@ -205,7 +205,7 @@ def wrap_session(config, doit):
             raise
         except Failed:
             session.exitstatus = EXIT_TESTSFAILED
-        except KeyboardInterrupt:
+        except (KeyboardInterrupt, exit.Exception):
             excinfo = _pytest._code.ExceptionInfo.from_current()
             exitstatus = EXIT_INTERRUPTED
             if initstate <= 2 and isinstance(excinfo.value, exit.Exception):