checkupdates: fix google play check method
checkupdates: fix google play check method

This has never worked with python3 and was also not properly adapted to
apps as objects. So we fix both of these issues.

f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver