On Python 2, there is no way to use a raw unicode string and represent `\u`, which is always interpreted as a unicode character, so we can't use raw strings here.  We could use byte strings, but then we'd have to have a mechanism to *not* do that on Python 3.

Python 2

```
>>> ur'\u'
  File "<stdin>", line 1
SyntaxError: (unicode error) 'rawunicodeescape' codec can't decode bytes in position 0-1: truncated \uXXXX
>>> ur'\\u'
u'\\\\u'
>>> u'\u'
  File "<stdin>", line 1
SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 0-1: truncated \uXXXX escape
>>> u'\\u'
u'\\u'
```

Python 3:

```
>>> r'\u'
'\\u'
>>> r'\\u'
'\\\\u'
>>> '\u'
  File "<stdin>", line 1
SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 0-1: truncated \uXXXX escape
>>> '\\u'
'\\u'
```