Add tox.ini for testing multiple runtimes

Author: rkoopmans

tox.ini

@@ -0,0 +1,8 @@
+[tox]
+envlist = py27,py36,py37,py38,py39,py310,py311,pypy2,pypy3
+
+[testenv]
+deps = -rtest-requirements.txt
+       -rrequirements.txt
+commands =
+    pytest {posargs}