Upgrade Python

[noyainrain]

Upgrade to Python 3.5 once it is available on Debian stable.

Make use of:

• https://docs.python.org/3/library/http.html#http.HTTPStatus
• https://docs.python.org/3/library/json.html#json.JSONDecodeError
• https://docs.python.org/3/whatsnew/3.5.html#pep-492-coroutines-with-async-and-await-syntax

Also interesting:

• https://docs.python.org/3/library/unittest.html#cmdoption-unittest--locals
• https://docs.python.org/3/whatsnew/3.5.html#email
• https://docs.python.org/3/whatsnew/3.5.html#smtplib
• https://docs.python.org/3/whatsnew/3.5.html#pep-484-type-hints