st_mtime_ns precision or rounding related test failure

[ThomasWaldmann]

Tests for PR #256 showed this test failure only on py 3.2, but the failure seems unrelated to the changes it the PR and might point to a general problem that doesn't show up always:

FAIL: test_overwrite (attic.testsuite.archiver.RemoteArchiverTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "attic/testsuite/archiver.py", line 324, in test_overwrite
    self.assert_dirs_equal('input', 'output/input')
  File "attic/testsuite/__init__.py", line 53, in assert_dirs_equal
    self._assert_dirs_equal_cmp(diff)
  File "attic/testsuite/__init__.py", line 84, in _assert_dirs_equal_cmp
    self.assert_equal(d1, d2)
AssertionError: Lists differ: ['dir2', 16832, 0, 0, 0, 2, 14... != ['dir2', 16832, 0, 0, 0, 2, 14...
First differing element 6:
1430088870571000000
1430088870570000000
- ['dir2', 16832, 0, 0, 0, 2, 1430088870571000000, {}]
?                                         ^
+ ['dir2', 16832, 0, 0, 0, 2, 1430088870570000000, {}]
?                                         ^