Fix conversions.py2to3 on unicode strings

[msullivan]

The introduction of string kinds in #49 missed updating the 2to3 visitor
for it. Update the visit_Str case to pass s.kind to Str.

Fixes #66.

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[gvanrossum]

LGTM!

[emmatyping]

LGTM

[msullivan]

Going to merge this. We should do another release soon.
(Though mypy doesn't actually need this fix)

[ilevkivskyi]

Thanks for fixing this! Having tests for this would be great, but the problem is that we dan't have any tests for typed_ast (probably this is another reason to merge it to CPython, that already has tons of tests).