Raise tox min version to avoid maximum recursion issue

There was regression in tox 2.2.0 for env-substitution
with setenv.

https://bitbucket.org/hpk42/tox/issues/99/environment-variable-usage-schema-env-xxx

It is fixed in 2.3.1
https://bitbucket.org/hpk42/tox/issues/285/tox-220-breaks-some-toxini-config-files

So no issue with 2.3.1, but let's raise the min version in tox.ini
to avoid maximum recursion issue.

Change-Id: Ie808e46a90a681b21c30998d2fc867981c2c05fd
Closes-Bug: #1534006
1 file changed