tox: add 38, 39

Signed-off-by: umarcor <unai.martinezcorral@ehu.eus>

tox.ini

diff --git a/tox.ini b/tox.ini
index ea11e92..0dab1bb 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,13 +1,17 @@
 [tox]
-envlist = py{27,34,35,36,37}
+envlist = py{27,34,35,36,37,38,39}
 
 [testenv]
 setenv =
     PYTHONPATH={toxinidir}/vtr_xml_utils
+
 basepython =
     py35: python3.5
     py36: python3.6
     py37: python3.7
+    py38: python3.8
+    py39: python3.9
+
 deps =
     check-manifest
     # If your project uses README.rst, uncomment the following:
@@ -15,14 +19,17 @@
     flake8
     pytest
     lxml
+
 commands =
     check-manifest --ignore tox.ini,tests*
     # This repository uses a Markdown long_description, so the -r flag to
     # `setup.py check` is not needed. If your project contains a README.rst,
     # use `python setup.py check -m -r -s` instead.
-    python setup.py check -m -s
-    flake8 .
-    pytest -vvvv
+    {envpython} setup.py check -m -s
+    {envpython} -m flake8 .
+    {envpython} -m pytest -vvvv
 [flake8]
+
 exclude = .tox,*.egg,build,data
+
 select = E,W,F