Restore diffcov

Author: jaraco

tox.ini

@@ -22,9 +22,10 @@ commands =
 	python -m sphinx . {toxinidir}/build/html
 
 [testenv:diffcov]
+deps =
+	diff-cover
 commands =
-	python -m coverage run -m pytest {posargs}
-	python -m coverage xml
+	pytest {posargs} --cov-report xml
 	diff-cover coverage.xml --html-report diffcov.html
 	diff-cover coverage.xml --fail-under=100