tox: add mypy-diff testenv

tox.ini

@@ -59,6 +59,15 @@ commands = pre-commit run --all-files --show-diff-on-failure {posargs:}
 extras = checkqa-mypy, testing
 commands = mypy {posargs:src testing}
 
+[testenv:mypy-diff]
+extras = checkqa-mypy, testing
+deps =
+    lxml
+    diff-cover
+commands =
+  -mypy --cobertura-xml-report {envtmpdir} {posargs:src testing}
+  diff-cover --fail-under=100 --compare-branch={env:DIFF_BRANCH:origin/{env:GITHUB_BASE_REF:master}} {envtmpdir}/cobertura.xml
+
 [testenv:docs]
 basepython = python3
 usedevelop = True