Apply correct Python version in tox base envs

Since we set basepython to python3 in tox's default testenv, this
has the unwanted side effect of causing all tox base envs (e.g.
py35) to use whatever interpreter is linked from the python3
executable name even if it isn't the requested version. Add a new
option from tox 3.1 to ignore this basepython override conflict.

diff --git a/tox.ini b/tox.ini
index 559af51..dce2bef 100644 (file)
--- a/tox.ini
+++ b/tox.ini
@@ -3,9 +3,10 @@
 # terms provided in the LICENSE file distributed with this software.
 
 [tox]
-minversion = 2.0
+minversion = 3.1
 envlist = dist, docs, flake8, yamllint, selftest_config, py3
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3