Two things changed in Python 3.4:

- 'basestring' is no longer defined, so use six.string_types

- True/False are now special AST node types (NamedConstant) rather than
  just names

(Good thing we had tests, or I wouldn't have noticed the 2nd thing!)

I found only one place where safe_eval() is called inside the ansible
codebase: in lib/template/__init__.py.  The call to safe_eval(result,
...) is protected by result.startswith('...'), which means result cannot
possibly be a byte string on Python 3 (or startswith() would raise, so
six.string_types (which excludes byte strings on Python 3) is fine here.