Fix CreateString with already-encoded string or bytearray in Python 2.7.

Fix CreateString with already-encoded string or bytearray in Python 2.7.

There was no way to pass an already-encoded string to
`builder.CreateString` in Python 2.7:

- Passing a `bytearray` raised a TypeError because `bytearray` was not
  recognized as an instance of `compat.binary_type`.
- Passing a utf-8 encoded `str` would cause the string to be
  double-encoded, because `compat.string_types = (basestring,)` and
  `basestring` is the base class of `str` and `unicode`, so the logic
  would never reach the `elif isinstance(s, compat.binary_type)` case.
- Converting a utf-8 encoded bytearray to `bytes` like
  `builder.CreateString(bytes(encoded_string))` does not work because
  in Python 2.7, bytes is just an alias for `str` so it behaves as
  above.

This change allows either `bytes` or `bytearray` as an already-encoded
string to be passed to `CreateString` in versions of Python that support
`bytearray`, and falls back to `str` in older versions.

In Python 2, it restricts unencoded string types to `unicode`, so `str`
can be used as an encoded, binary representaiton.