Add minversion to tox.ini (#3292)

Cryptography uses new features from tox 2.4. Tox 2.3 happily ignores the
new config stanzes and doesn't install dependency. This can lead to strange
test failures.

With minversion=2.4, tox 2.3 fails to run properly:
$ tox
ERROR: tox version is 2.3.1, required is at least 2.4

Signed-off-by: Christian Heimes <christian@python.org>
1 file changed