Bump min tox version to 3.18.0

Let's bump minimal tox version so that we can rename
whitelist_externals option to allowlist_externals one.

https: //tox.wiki/en/latest/changelog.html#v3-18-0-2020-07-23
Change-Id: I4b79e36b5ee3aefc59d262abe443271ad0dec100

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 1.6
+minversion = 3.18.0
 skipsdist = True
 envlist = bashate
 
@@ -9,7 +9,7 @@ install_command = pip install {opts} {packages}
 
 [testenv:bashate]
 deps = bashate
-whitelist_externals = bash
+allowlist_externals = bash
 commands = bash -c "find {toxinidir}          \
          -not \( -type d -name .?\* -prune \) \
          -not \( -type d -name doc -prune \)  \
@@ -31,4 +31,4 @@ deps =
 commands =
   rm -rf doc/build
   sphinx-build -W -b html doc/source doc/build/html
-whitelist_externals = rm
+allowlist_externals = rm