move apk signature verification into getsig() function
move apk signature verification into getsig() function

This makes the python replacement behave like an all-in-one replacement
for getsig.java.

fixes #5 https://gitlab.com/fdroid/fdroidserver/issues/5

OneEducation/AppUniverse_Server,matlink/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,matlink/fdroidserver,OneEducation/AppUniverse_Server,f-droid/fdroid-server,f-droid/fdroidserver,OneEducation/AppUniverse_Server,matlink/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,OneEducation/AppUniverse_Server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,matlink/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,OneEducation/AppUniverse_Server,fdroidtravis/fdroidserver,matlink/fdroidserver,fdroidtravis/fdroidserver