https://github.com/miyuchina/mistletoe/issues/220

--- mistletoe-1.4.0/tox.ini.orig
+++ mistletoe-1.4.0/tox.ini
@@ -1,3 +1,7 @@
+[testenv]
+commands =
+    python -m pytest {posargs}
+
 [flake8]
 # See https://www.flake8rules.com/ for the full list of error codes.
 extend-ignore = E124,E126,E127,E128,E501