Update APNs
This PR updates apntool.py
for python3, updates APNs DB from CM 13 (https://github.com/CyanogenMod/android_vendor_cm/blob/ef1a4793184e20551632955deaee8dc8edf3efee/prebuilt/common/etc/apns-conf.xml) and fixes apntool.py
to avoid errors while parsing this new XML.
May fix #367 (closed).