test: restore tox.ini, it is useful to test things locally [ci skip]

Author: ntamas

tox.ini

@@ -0,0 +1,27 @@
+# Tox (http://tox.testrun.org/) is a tool for running tests
+# in multiple virtualenvs. This configuration file will run the
+# test suite on all supported python versions. To use it, "pip install tox"
+# and then run "tox" from this directory.
+
+[tox]
+envlist = py36, py37, py38, py39, pypy, pypy3
+
+[gh-actions]
+python =
+    3.6: py36
+    3.7: py37
+    3.8: py38
+    3.9: py39
+    pypy-3.7: pypy3
+
+[testenv]
+commands = python -m unittest
+deps =
+    scipy; platform_python_implementation != "PyPy"
+    numpy; platform_python_implementation != "PyPy"
+    networkx; platform_python_implementation != "PyPy"
+    pandas; platform_python_implementation != "PyPy"
+passenv = PATH
+setenv =
+    TESTING_IN_TOX=1
+