Fix running nose with xunit.

When running nosetest with xunit, the tests would crash for external
projects using python 3.5 with something similar to:

Traceback (most recent call last):
  File "./lib/python3.5/site-packages/luigi/mock.py",
  line 128, in write
      stderrbytes = sys.stderr.buffer
      AttributeError: 'Tee' object has no attribute 'buffer'

This is becuase xunit overrides sys.stderr, and the class it uses does
not have a 'buffer' attribute.

Nose source:
  https://github.com/nose-devs/nose/blob/master/nose/plugins/xunit.py#L127-L147

luigi/mock.py

@@ -124,15 +124,10 @@ def set_wrapper(self, wrapper):
                 self.wrapper = wrapper
 
             def write(self, data):
-                if six.PY3:
-                    stderrbytes = sys.stderr.buffer
-                else:
-                    stderrbytes = sys.stderr
-
                 if mock_target._mirror_on_stderr:
                     if self._write_line:
                         sys.stderr.write(fn + ": ")
-                    stderrbytes.write(data)
+                    sys.stderr.write(data)
                     if (data[-1]) == '\n':
                         self._write_line = True
                     else: