test: convert to pytest

Instead of built-in `unittest`, now use `pytest`. Find versions that work, convert tests.

ref: N25B-168

requirements.txt

@@ -1,3 +1,5 @@
 pyzmq<16
 pyaudio<=0.2.11
-mock~=3.0.5
+pytest<5
+pytest-mock<3.0.0
+pytest-cov<3.0.0