fix: Test coverage broken by migration from setup.py to pyproject.toml

`setup.py test` has been deprecated for years. See:

- pypa/setuptools#1684
- pypa/setuptools#4522

Makefile

@@ -101,7 +101,7 @@ test-all: ## run tests on every Python version with tox
 	tox
 
 test-coverage: ## run tests and record test coverage
-	coverage run --rcfile=setup.cfg setup.py test
+	coverage run --rcfile=setup.cfg -m unittest discover -v -c -b -s src -t src
 
 test-coverage-report: test-coverage-report-console
 test-coverage-report: test-coverage-report-xml

tox.ini

@@ -7,7 +7,7 @@ envlist =
 [testenv]
 setenv =
     PYTHONPATH = {toxinidir}:{toxinidir}/cl_sii
-commands = coverage run --rcfile=setup.cfg setup.py test
+commands = coverage run --rcfile=setup.cfg -m unittest discover -v -c -b -s src -t src
 deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/requirements-dev.txt