py-ZopeTestrunner: updated to 5.1

5.1:
- Recover more gracefully when layer setUp or tearDown fails, producing
  useful subunit output.
- Prevent a spurious warning from the ``--require-unique`` option if the
  ``--module`` option was not used.
- Add optional buffering of standard output and standard error during tests,
  requested via the ``--buffer`` option or enabled by default for subunit.
- Fix incorrect failure counts in per-layer summary output, broken in 4.0.1.

5.0:
- Fix test failures and deprecation warnings occurring when using Python 3.8a1.
- Drop support for Python 3.4.

(adam)