make it compatible with Python 2.7

[j-ed]

Hi @hannob I like to run your program on one of my servers which has Python 2.7 installed by default.
Now I wonder if it would be possible to make it compatible with this version. I already did some tests on my own but without success (I'm not a Python expert). The following error is shown if I execute it:

...
RSA e: 0x10001
Modulus size: 2048 bits, 256 bytes
Traceback (most recent call last):
  File "./robot-detect", line 184, in <module>
    pms_good = int(gmpy2.powmod(pms_good_in, e, N)).to_bytes(modulus_bytes, byteorder="big")
AttributeError: 'long' object has no attribute 'to_bytes'

From what I understood a to_bytes attribute doesn't exist for that version, but people seem to have discussed this kind of problem already and provided possible workarounds: therealmik/pyrdiff#1

[hannob]

I have no interest in making the code less readable to make it compatible with outdated python versions, sorry. You don't have to run the code on the server you're testing.