tox: add a gitlint environment

The new gitlint tox environment is not run by default, it can be run
manually or by CI in order to check that commit messages meet some basic
quality criteria.

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

tox.ini

@@ -60,3 +60,11 @@ deps =
     PyYAML
 commands =
     python -m sambacc.schema.tool
+
+# this gitlint rule is not run by default.
+# Run it manually with: tox -e gitlint
+[testenv:gitlint]
+deps =
+    gitlint==0.19.1
+commands =
+    gitlint --commits origin/master.. lint