Turn off tftpy logging

[Cable-1406]

I am trying to use this library in a larger project involving downloading from different hosts with different protocols and importantly with my own logging concept. The included logs of tftpy are kind of "in the way" with that as my screen gets spammed with "timeout", "resending", "waiting" and so on messages if multiple tftp clients are simply unreachable (which can be normal in my case).

Thanks for your reply in advance :)