Use tools from virtualenv for test coverage.

Author: mikaelfrykholm

Makefile

@@ -9,10 +9,10 @@ quick_test:
 	PYFF_SKIP_SLOW_TESTS=1 PYTHONPATH=$(SRCDIR) pytest
 
 test_coverage:
-	coverage erase
-	PYTHONPATH=$(SRCDIR) pytest --cov=src/pyff
+	python -m coverage erase
+	python -m coverage run -m pytest --cov=src/pyff
 	mv .coverage .coverage.1
-	coverage combine
+	python -m coverage combine
 
 reformat:
 	ruff --format $(SOURCE)