Changes

configure: set min python version check to 3.5

The osmo-gsm-tester and 201705 jobs, which use the OE poky SDK from 2017
have started failing since the check for python 3.6 was added. This SDK
has version 3.5, so lower the required version check to that for now.

Fixes: 489a6f ("configure.ac: check for Python interpreter unconditionally")
Change-Id: Ia39e7e04dd8a31efaf87fd1f0fbbfdc5b5fa37c2