py3: conditionalize test-lfs-serve-access.t for Python 3.8 This is another case where Python 3.8's traceback printing varies subtly from older Python versions. Again, I'm not sure why. But this is apparently the new behavior. With this change, test-lfs-serve-access.t now passes on Python 3.8! Differential Revision: https://phab.mercurial-scm.org/D7947