Add codecov coverage

tox.ini

@@ -5,6 +5,7 @@ envlist =
     py{35,36,37,38,39}-django22
     py{36,37,38,39}-django{30,31,32}
     lint
+    coverage
 
 [gh-actions]
 python =
@@ -38,3 +39,10 @@ commands =
     flake8 django_migration_linter/
     black --check django_migration_linter/ tests/ manage.py setup.py
     isort --check-only .
+
+[testenv:coverage]
+commands =
+    coverage run manage.py test --no-input {posargs}
+    coverage xml -i
+extras = test
+pip_pre = true