pythonPackages.pypeg2: fix nondeterministic test failure

If test_xmlast runs before test_pyPEG2, the tests fail with:

ERROR: runTest (pypeg2.test.test_pyPEG2.ParseInvisibleTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/build/pyPEG2-2.15.2/pypeg2/test/test_pyPEG2.py", line 258, in runTest
    self.assertEqual(r._ignore1, None)
AttributeError: 'C1' object has no attribute '_ignore1'

https://hydra.nixos.org/build/74632015