Fix outdated tox minversion

allowlist_externals option was added in 3.18.0.

Change-Id: If1cf4a63257b8182b6c1eb72dbf35077275cb05b
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.2.1
+minversion = 3.18.0
 envlist = py3,pep8,alint,cover
 skipsdist = True
 ignore_basepython_conflict=true
@@ -103,4 +103,4 @@ deps = codespell
 # note(JayF): {posargs} lets us run `tox -ecodespell -- -w` to get codespell
 #             to correct spelling issues in our code it's aware of.
 commands =
-  codespell {posargs}
+  codespell {posargs}