tests / tox: Use pytest instead of unittest

Tests can still be run using unittest (and are in the test:3.7 gitlab
CI item), but this gives prettier output on failures.

Simplifications from moving away from unittest toward pytest can be
explored independently of this change.

Author: chrysn

tox.ini

@@ -9,10 +9,11 @@ envlist = {py37,py38,py39,py310,pypy3}-{noextras,allextras},py38-uvloop
 [testenv]
 deps =
     coverage
+    pytest
 
 commands =
     python -m coverage run --parallel -m aiocoap.cli.defaults
-    python -m coverage run --parallel -m unittest discover -v {posargs}
+    python -m coverage run --parallel -m pytest {posargs}
 
 [testenv:{py37,py38,py39,py310,pypy3}-noextras]
 description = Running on plain Python