TST: fix test error due to 2to3 conversion on py3.3.3

This fixes one test error when building and testing numpy 1.7.x on Python 3.3.3:
```
======================================================================
FAIL: test_decode (test_defchararray.TestMethods)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "X:\Python33-x32\lib\site-packages\numpy\core\tests\test_defchararray.py", line 319, in test_decode
    assert_(A.decode('unicode-escape')[0] == '\\u03a3')
  File "X:\Python33-x32\lib\site-packages\numpy\testing\utils.py", line 35, in assert_
    raise AssertionError(msg)
AssertionError
```

Apparently Python 3.3.3 converts `assert_(A.decode('unicode-escape')[0] == '\u03a3')` to `assert_(A.decode('unicode-escape')[0] == '\\u03a3')`