Correct syntax for invoking pytest

Author: jaraco

tox.ini

@@ -23,7 +23,7 @@ commands =
 
 [testenv:diffcov]
 commands =
-	python -m coverage run pytest {posargs}
+	python -m coverage run -m pytest {posargs}
 	python -m coverage xml
 	diff-cover coverage.xml --html-report diffcov.html
 	diff-cover coverage.xml --fail-under=100