Add support for Python3

[user2684]

Describe the solution you'd like
Switch to python3

[user2684]

Requirements

• Backwards compatibility with packages using python2
• Interpretability of packages using different languages

Design

• Python3 is added ON TOP of python2 within the SDK so to allow packages using python2 to continue working
• A new folder called "python3" is added with the new code. To run a module code with python3, this has to be run with python3 -m sdk.python3.module.start
• A new setting called "language" is added to the manifest (without changing the manifest schema). Docker entrypoint script searches for this value, If not present, python2 is used, if set the provided language is used (at this time only python3 but this opens up to other languages like go).

Other Changes

• A few changes have been introduced for python3 SDK to deal with bytes to be decoded into strings
• Resin raspbian based image (containing a too old version of python3. - 3.4) has been migrated to balena rpi-raspbian image