Add "tox -e mypy-diff"

Not done by default with "tox -e mypy" because it disables mypy's caching,
causing its runtime to go from <1s to ~15s:
https://github.com/python/mypy/issues/9041

See #1456

misc/requirements/requirements-mypy.txt

@@ -1,8 +1,16 @@
 # This file is automatically generated by scripts/dev/recompile_requirements.py
 
+diff-cover==3.0.1
+inflect==4.1.0
+Jinja2==2.11.2
+jinja2-pluralize==0.3.0
 lxml==4.5.1
+MarkupSafe==1.1.1
 mypy==0.782
 mypy-extensions==0.4.3
+pluggy==0.13.1
+Pygments==2.6.1
 -e git+https://github.com/stlehmann/PyQt5-stubs.git@master#egg=PyQt5_stubs
+six==1.15.0
 typed-ast==1.4.1
 typing-extensions==3.7.4.2

misc/requirements/requirements-mypy.txt-raw

@@ -1,5 +1,6 @@
 mypy
 lxml  # For HTML reports
+diff-cover
 -e git+https://github.com/stlehmann/PyQt5-stubs.git@master#egg=PyQt5-stubs
 
 # remove @commit-id for scm installs

tox.ini

@@ -200,6 +200,15 @@ deps =
 commands =
     {envpython} -m mypy qutebrowser tests {posargs}
 
+[testenv:mypy-diff]
+basepython = {env:PYTHON:python3}
+pip_version = pip
+passenv = {[testenv:mypy]passenv}
+deps = {[testenv:mypy]deps}
+commands =
+    {envpython} -m mypy --cobertura-xml-report {envtmpdir} qutebrowser tests {posargs}
+    {envdir}/bin/diff-cover --fail-under=100 --compare-branch={env:DIFF_BRANCH:origin/{env:GITHUB_BASE_REF:master}} {envtmpdir}/cobertura.xml
+
 [testenv:sphinx]
 basepython = {env:PYTHON:python3}
 pip_version = pip