Migrate from gbulb to asyncio-glib

[chrysn]

As gbulb is abandoned, https://github.com/jhenstridge/asyncio-glib/ seems to be the most direct successor.

This will not actually require code changes to aiocoap itself, but to examples, documentation and the test suite.

[chrysn]

This is made more urgent by the apparent incompatibility between Python 3.8 and gbulb.

[chrysn]

This is blocked on two issues:

• https://github.com/jhenstridge/asyncio-glib/pull/7 or similar needs to be merged in -- otherwise examples like the widgets demo won't react to user interaction.
• At way more places than is good for the library it creates a Future without going through the loop (where ideally it should create as little futures as possible). Until those are switched to be created from the loop, those are not asyncio-glib futures, and thus don't work well with it (creating the same symptom as the above, just not fixable cleanly).