py3: byteify the LFS blobstore module This is almost entirely b'' prefixing, with a couple of exceptions forced to bytes. Much of this is also borrowed from Augie's code. There's an HTTPError.read() that I flagged that I assume needs to be converted to bytes, but I can't find confirmation. Handling the deserialized JSON object over several functions made r'' vs b'' accesses confusing, so this assumes that the JSON object will be converted to bytes immediately. That will be done in the following commits, so it's not buried in these trivial changes.