tox: Replace py344 with py34

tox does not know what "py344" is and just runs the tests on your
default Python version instead.

For a complete list of default enviroments, see
https://tox.readthedocs.io/en/latest/example/basic.html#a-simple-tox-ini-default-environm

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist =
-    py344,
+    py34,
     py35,
     coverage,
     #flake8,