Add mypy-diff to tox/CI

.github/workflows/main.yml

@@ -189,3 +189,19 @@ jobs:
       run: |
         sudo apt-get install pandoc
         tox -e publish-gh-release-notes
+
+  mypy-diff:
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - name: Set up Python
+      uses: actions/setup-python@v1
+      with:
+        python-version: "3.8"
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install --upgrade wheel setuptools tox
+    - name: tox -e mypy-diff
+      run: "tox -e mypy-diff"

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