Send data to the subprocess' stdin.

        Returns the number of bytes written.

def send(self, s):
        '''Send data to the subprocess' stdin.

        Returns the number of bytes written.
        '''
        s = self._coerce_send_string(s)
        self._log(s, 'send')

        b = self._encoder.encode(s, final=False)
        if PY3:
            return self.proc.stdin.write(b)
        else:
            # On Python 2, .write() returns None, so we return the length of
            # bytes written ourselves. This assumes they all got written.
            self.proc.stdin.write(b)
            return len(b)