Upgrade existing code to Python 3.6

[geotom]

Tasks

• Change normal strings to formatted literals

• Annotate functions and variables with type hints

• Introduce pathlib instead of string pathes

• Check if we can make use of asynchronous generators/comprehensions

Todo

• Correct the store error (path)
• Check if tests work afterwards

[geotom]

Missing annotations in

• daemons.py
• client.py
• node.py
• object.py (data)
• exceptions.py

[geotom]

Regarding pathlib: https://treyhunner.com/2019/01/no-really-pathlib-is-great/