Fixes #23028: inspectdb should handle unique_together

`strip_prefix` will not work for a tuple where `u'` happens several times, that's really some over-engineering there for a testcase that seems to work fine?

```
>>> def test1():
...     unique_together = []
...     fields = ['a', 'b']
...     unique_together.append(tuple(fields))
...     return '%s' % tuple(unique_together)
... 
>>> from __future__ import unicode_literals
>>> 
>>> def test2():
...     unique_together = []
...     fields = ['a', 'b']
...     unique_together.append(tuple(fields))
...     return '%s' % tuple(unique_together)
... 
>>> test1(), test2()
("('a', 'b')", u"(u'a', u'b')")
```