update our tox version

the tox version we used are too old.

Change-Id: I661918c66aca69d9e99732d0f5910687e39b8787
Signed-off-by: song baisen <songbaisen@szzt.com.cn>
Co-Authored-By: tangzhuo <ztang@hnu.edu.cn>, zhiyuan_cai <luckyvega.g@gmail.com>

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 1.6
+minversion = 2.0
 envlist = py36,py27,pypy,pep8
 skipsdist = True