deprecation warnings in compact.py under Python 3.5

[terwilliger42]

I'm getting deprecation warnings in compact.py when running my unittests on Python 3.5. Will submit a PR.

.../python3.5/site-packages/thriftpy/protocol/compact.py:419: DeprecationWarning: tostring() is deprecated. Use tobytes() instead.
  write_varint(self.trans, make_zig_zag(i32, 32))
.../python3.5/site-packages/thriftpy/protocol/compact.py:339: DeprecationWarning: tostring() is deprecated. Use tobytes() instead.
  write_varint(self.trans, i32)
...../python3.5/site-packages/thriftpy/protocol/compact.py:416: DeprecationWarning: tostring() is deprecated. Use tobytes() instead.
  write_varint(self.trans, make_zig_zag(i16, 16))
.../python3.5/site-packages/thriftpy/protocol/compact.py:422: DeprecationWarning: tostring() is deprecated. Use tobytes() instead.
  write_varint(self.trans, make_zig_zag(i64, 64))