NOT_DONE_YET comparison warning

[arnimarj]

It seems that https://github.com/twisted/klein/blob/master/src/klein/_resource.py#L278 is doing a value comparison with NOT_DONE_YET, similarly to twisted.web in https://twistedmatrix.com/trac/ticket/9597

I get these warning regularly: BytesWarning: Comparison between bytes and int when running in bytes-warning mode (-b)