General cleanup
General cleanup

  * Cleanup of Comments
  * Remove len() when evaluating for an empty list
  * Remove extra callback in basic_cancel

jstnlef/pika,vrtsystems/pika,reddec/pika,Tarsbot/pika,knowsis/pika,pika/pika,benjamin9999/pika,shinji-s/pika,Zephor5/pika,hugoxia/pika,fkarb/pika-python3,vitaly-krugl/pika,zixiliuyue/pika,skftn/pika,renshawbay/pika-python3