bump tox to 3.1.1

Change-Id: I73576906f64cd76fde85c678e2a26e80f0efbe25

tox.ini

@@ -1,14 +1,14 @@
 [tox]
-minversion = 1.6
+minversion = 3.1.1
 skipsdist = True
 envlist = bashate
+ignore_basepython_conflict = true
 
 [testenv]
+basepython = python3
 usedevelop = False
-install_command = pip install {opts} {packages}
 
 [testenv:bashate]
-basepython = python3
 deps = bashate
 whitelist_externals = bash
 commands = bash -c "find {toxinidir}          \
@@ -23,4 +23,4 @@ commands = bash -c "find {toxinidir}          \
           -name functions\* -or               \
           -wholename \*/lib/\*                \
          \)                                   \
-         -print0 | xargs -0 bashate -v -iE006"
+         -print0 | xargs -0 bashate -v -iE006"