package/i2c-tools: bump to version 3.1.2

This patch bumps version to 3.1.2 and adds support for the recently
introduced python3 support for py-smbus.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>