Define basestring in Bio._py3k
Use basestring from Bio._py3k

The goal here is to be able to use the old Python 2
style 'basestring' under both Python 2 and Python 3,
and thus avoids needing to use the 2to3 basestring
fixer, bringing us closer to not needing 2to3 at all.
Disable 2to3 basestring fixer, using Bio._py3k instead