Depends on #86

Description

• uses https://github.com/chrysn/aiocoap instead of https://github.com/Tanganelli/CoAPthon3
• added monkeypatch from here https://github.com/betaboon/aioairctrl/blob/master/aioairctrl/coap/aiocoap_monkeypatch.py (see Question to aiocoap betaboon/aioairctrl#3), needed for some devices

Advantages

• aiocoap seems to be better supported, used more often (more commits, stars, forks)
• newer pypi-package (Februar 2021 opposed to January 2018)
• we had some bigger troubles with coapthon3, therefore needed to clone this, seems that this library is more stable.

Disadvantages

• need to change infrastructure, since it uses asyncio
• lowest version would be python 3.7, here we are using 3.4 or higher, so we would need to change this requirement

Disclaimer

This is just a spike working with coap only:
- no adjustments to http and plain_coap (need to change to asyncio)
- no adjustments on base-classes (changed coap-class directly)
- no adjustments to tests

For coap it works like it worked with Coapthon3, some work will still be needed, just not sure if we should make this step, I think I'll make some changes like mqtt + usage of observable before.
Therefore I made only a draft PR.