Specify we need virtualenv>20,<21

Signed-off-by: Bernat Gabor <bgabor8@bloomberg.net>

tox.ini

@@ -1,18 +1,24 @@
-# 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.
-
+# http://tox.readthedocs.org/ - sets up and runs the test suite based on a declarative configuration
 [tox]
-envlist = py27, py34, py35, py36, py37, py38
+envlist =
+  py39
+  py38
+  py37
+  py36
+  py35
+  py34
+  py27
+  pypy3
+  pypy2
 
 [testenv]
+description = run test suite under {basepython}
 commands =
-    pytest {posargs:-vv}
+  pytest {posargs:-vv}
 deps =
-    tox>=3.0.0
-    graphviz
-    pip>=8.0.2
-    pytest
-    pytest-cov
-    virtualenv>=20
+  graphviz
+  pip>=8.0.2
+  pytest
+  pytest-cov
+  virtualenv>=20,<21
+  mock;python_version<"3"