bpo-37053: handle strings like u"bar" correctly in Tools/parser/unparse.py (GH-13583)

Constant.kind is added in https://bugs.python.org/issue36280.
Current possible values for Constant.kind are "u" or None.

For r'bar' and b'bar', Constant.kind value is None, so there's no need
for special handling.


https://bugs.python.org/issue37053