2to3: Apply unicode fixer.

The unicode fixer strip the u from u'hi' and converts the unicode type
to str. The first won't work for Python 2, so unicode from numpy.compat
is used instead of the u prefix. The second fix is also implemented by
importing the unicode type from numpy.compat. The result is that the
official unicode fixer is not actually used for anything.

Closes #3089.