Since app engine deploy is just a python script, why can't you just write a shell script that calls 'git push' followed by 'python appcfg.py deploy'?
Any bitbucket hooks that will send from bitbucket->appengine after you upload to app is probably a bad idea since it will require storing your app engine login credentials on github.
Other projects might have your app server pull from github/bitbucket. You can do this if your app engine site just serves static websites using http://drydrop.binaryage.com/, but you can't update actual running code this way.