Yay! With the current version on Github I'm getting nice beeps from background radiation! The calibrate does not give me a plateau I was expecting though:

>>> geiger.calibrate()
Calibrating HV in 7 steps of 10 seconds (70s total) from duty=20 to duty=90
HV pin 13, freq 10kHz, duty 20, event pin 5
With PWM duty 20 we had 0 counts in 10 seconds
HV pin 13, freq 10kHz, duty 30, event pin 5
With PWM duty 30 we had 0 counts in 10 seconds
HV pin 13, freq 10kHz, duty 40, event pin 5
With PWM duty 40 we had 3 counts in 10 seconds
HV pin 13, freq 10kHz, duty 50, event pin 5
With PWM duty 50 we had 1 counts in 10 seconds
HV pin 13, freq 10kHz, duty 60, event pin 5
With PWM duty 60 we had 5 counts in 10 seconds
HV pin 13, freq 10kHz, duty 70, event pin 5
With PWM duty 70 we had 4 counts in 10 seconds
HV pin 13, freq 10kHz, duty 80, event pin 5
With PWM duty 80 we had 5 counts in 10 seconds
>>> 

So apparently I'm not able to get the tube in continuous discharge mode. This might as well be a limitation of the HV part of the circuit though. Later today I'll post a nice picture in this log of the whole thing on the breadboard, and the next step is to try to calibrate it using a radioactive source. (I've heard that fresh rain is an OK source).