Use Pyjnius
A little Android Googling finds the Java. Then you read enough about Pyjnius above to figure out how to encapsulate in Python.
It might be something like this. BUT I didn't test it and I have no idea if it works.
from android import mActivity, autoclass
Context = autoclass('android.content.Context')
strength = mActivity.getSystemService(Context.TELEPHONY_SERVICE)
.getSignalStrength()