Revert "Remove tox.ini"

This reverts commit 61db7c3.

I use tox for my local testing during isso development.

Author: jelmer

tox.ini

@@ -0,0 +1,20 @@
+[tox]
+envlist = py35,py36,py37,py38,py39,py310
+
+[testenv]
+deps =
+    pytest
+    pytest-cov
+    setuptools>=6.1
+commands =
+    pytest isso/
+
+[testenv:debian]
+deps=
+    itsdangerous
+    Jinja2
+    misaka>=2.0,<3.0
+    html5lib
+    werkzeug>=1.0
+    bleach
+    flask-caching>=1.9