Where is the mistake in my RSA by-hand calculation?
Your $d$ is indeed incorrect. I get $d = 4235309647073$, using Wolfram alpha.

As to python, use the builtin pow function with a third argument equal to the modulus.

So 


message = 6
encmessage = pow(message, 17, 6000029000033)
assert message == pow(encmessage, 4235309647073, 6000029000033) 

which will apply a smart algorithm like those in answers to this question. Never compute the direct power first!