Fix CI warning in diffcov report (jaraco/skeleton#194)

UserWarning: The --html-report option is deprecated. Use --format html:diffcov.html instead.

Author: DimitriPapadopoulos

tox.ini

@@ -20,7 +20,7 @@ deps =
 	diff-cover
 commands =
 	pytest {posargs} --cov-report xml
-	diff-cover coverage.xml --compare-branch=origin/main --html-report diffcov.html
+	diff-cover coverage.xml --compare-branch=origin/main --format html:diffcov.html
 	diff-cover coverage.xml --compare-branch=origin/main --fail-under=100
 
 [testenv:docs]