Fix python PoW
Fix python PoW

- recent changes cause the pool workers to trigger shutdown and freeze.
  Now python PoW works again

hb9kns/PyBitmessage,debguy0x/PyBitmessage,bmng-dev/PyBitmessage,debguy0x/PyBitmessage,debguy0x/PyBitmessage,debguy0x/PyBitmessage,bmng-dev/PyBitmessage,hb9kns/PyBitmessage,timothyparez/PyBitmessage,bmng-dev/PyBitmessage,timothyparez/PyBitmessage,hb9kns/PyBitmessage,timothyparez/PyBitmessage,hb9kns/PyBitmessage,timothyparez/PyBitmessage