Run coverage analysis locally, too.

tox.ini

@@ -1,15 +1,25 @@
 [tox]
 envlist =
-    flake8, py27, pypy, py35, py36
+    flake8, py27, pypy, py35, py36, py37, py38, pypy3, coverage
 
 [testenv]
 commands =
     zope-testrunner --test-path=src []
-deps =
-   .[test,zcml,extract]
+extras =
+    test
+    zcml
+    extract
 
 [testenv:flake8]
 basepython = python3.6
 skip_install = true
 deps = flake8
 commands = flake8 --doctests src setup.py
+
+[testenv:coverage]
+basepython = python3.6
+usedevelop = true
+commands =
+    coverage run -m zope.testrunner --test-path=src []
+    coverage report --show-missing --fail-under=89
+deps = coverage