tox: Fix broken autopep8 target

Change-Id: I9c9e14be32b8c6e0d668741cbd6d6821bdb4392e
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
diff --git a/tox.ini b/tox.ini
index 0fbc252..aa3be46 100644
--- a/tox.ini
+++ b/tox.ini
@@ -395,6 +395,8 @@
 
 [testenv:autopep8]
 deps = autopep8>=2.1.0
+allowlist_externals =
+    {toxinidir}/tools/format.sh
 commands =
     {toxinidir}/tools/format.sh