Update coverage from 5.5 to 6.1.1

docs/requirements.txt

@@ -20,7 +20,7 @@ click==8.0.1
     # via pip-tools
 colorama==0.4.4
     # via sphinx-autobuild
-coverage==5.5
+coverage==6.1.1
     # via -r requirements.in
 datetime==4.3
     # via -r requirements.in