add coverage reporting

tox.ini

@@ -1,14 +1,29 @@
 [tox]
-envlist = py27,py35,py36,py37,py38,py39,py310,py311,py312,mypy,mypy2
+envlist = clean,py27,py35,py36,py37,py38,py39,py310,py311,py312,mypy,mypy2
 
 [testenv]
 deps = -rrequirements_dev.txt
 commands =
-    pytest {posargs}
+    pytest --cov --cov-append --cov-report=term-missing {posargs}
+depends =
+    {py27,py312}: clean
+    report: py27,py312
 
 [testenv:mypy]
 basepython = python3.12
 deps =
     mypy
     -rrequirements_dev.txt
 commands = mypy cert_chain_resolver
+
+[testenv:report]
+deps = coverage
+skip_install = true
+commands =
+    coverage report
+    coverage html
+
+[testenv:clean]
+deps = coverage
+skip_install = true
+commands = coverage erase