Normalize recent GNU ls output to match the expected BSD-style output present...

Normalize recent GNU ls output to match the expected BSD-style output present in the docstrings of subprocess.py under Python 3. Fixes #681.

greentest/test__doctests.py

 from __future__ import print_function
-import sys
+import doctest
+import functools
 import os
 import re
-import doctest
-import unittest
+import sys
 import traceback
+import unittest
+
 import gevent
 from gevent import socket
 from greentest import walk_modules
@@ -16,6 +18,25 @@ def myfunction(*args, **kwargs):
     pass
 
 
+class RENormalizingOutputChecker(doctest.OutputChecker):
+    """
+    Pattern-normalizing output checker. Inspired by one used in zope.testing.
+    """
+
+    def __init__(self, patterns):
+        self.transformers = [functools.partial(re.sub, replacement) for re, replacement in patterns]
+
+    def check_output(self, want, got, optionflags):
+        if got == want:
+            return True
+
+        for transformer in self.transformers:
+            want = transformer(want)
+            got = transformer(got)
+
+        return doctest.OutputChecker.check_output(self, want, got, optionflags)
+
+
 if __name__ == '__main__':
     cwd = os.getcwd()
     try:
@@ -42,6 +63,13 @@ if __name__ == '__main__':
             sys.exit('No modules found matching %s' % ' '.join(allowed_modules))
 
         suite = unittest.TestSuite()
+        checker = RENormalizingOutputChecker((
+            # Normalize subprocess.py: BSD ls is in the example, gnu ls outputs
+            # 'cannot access'
+            (re.compile('cannot access non_existent_file: No such file or directory'),
+             'non_existent_file: No such file or directory'),
+        ))
+
         tests_count = 0
         modules_count = 0
         for m, path in sorted(modules):
@@ -49,7 +77,7 @@ if __name__ == '__main__':
                 contents = f.read()
             if re.search(br'^\s*>>> ', contents, re.M):
                 try:
-                    s = doctest.DocTestSuite(m, extraglobs=globs)
+                    s = doctest.DocTestSuite(m, extraglobs=globs, checker=checker)
                     test_count = len(s._tests) # pylint: disable=W0212
                     print('%s (from %s): %s tests' % (m, path, test_count))
                     suite.addTest(s)