tests for different versions

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35, py36, py37, py38, flake8
+envlist = py27, py30, py31, py32, py33, py34, py35, py36, py37, py38, flake8
 
 [travis]
 python =
@@ -7,6 +7,12 @@ python =
     3.7: py37
     3.6: py36
     3.5: py35
+    3.4: py34
+    3.3: py33
+    3.2: py32
+    3.1: py31
+    3.0: py30
+    2.7: py27
 
 [testenv:flake8]
 basepython = python
@@ -20,7 +26,7 @@ deps =
     -r{toxinidir}/requirements_dev.txt
 ; If you want to make tox run the tests with the same versions, create a
 ; requirements.txt with the pinned versions and uncomment the following line:
-;     -r{toxinidir}/requirements.txt
+     -r{toxinidir}/requirements.txt
 commands =
     pip install -U pip
     pytest --basetemp={envtmpdir}