Importing python module should not trigger package installation

[pbrkr]

After the last commit (7b1ffcc), import of sdk_utils or anything which depends on it (tb_device_mqtt & tb_gateway_mqtt) will result in attempted installation of mmh3/pymmh3 via pip. This is counter-intuitive and surprising - importing a Python module should not have side effect which modify the system state.

Furthermore, this makes the tb-mqtt-client package unusable on embedded systems which either lack pip (to save space), have a read-only rootfs or lack internet access. Automated installation of packages at runtime also breaks installation reproducibility (as there's no way to control which version of the dependency is installed) and defeats attempts to achieve license compliance.

Dependencies should be expessed in setup.py so that they can be installed in the usual way.