A standalone test case for the API thread

[g1itch]

Hello!

I've started the standalone testcase for the API thread. IMHO it's the proper way to test the commands, which are not closely tied to the rest parts of PyBM (SQL, AddressGenerator, etc.). And it also brings the api module closer to the python3 compatibility.

So far it looks broken in the same way as #1895, but I think that merging the subset of my network branch may help better here. I'm going to write some tests for the network soon.