Fix outdated tox minversion

allowlist_externals is supported since 3.18.0 . Also fix the old py37
env because python 3.7 is no longer supported.

Change-Id: I23aef6b287eedd74edba802b3dc0027b9bb3c7b3

tox.ini

@@ -1,6 +1,6 @@
 [tox]
-minversion = 3.1.1
-envlist = py37,pep8
+minversion = 3.18.0
+envlist = py3,pep8
 ignore_basepython_conflict = True
 
 [testenv]
@@ -16,7 +16,7 @@ deps =
        -r{toxinidir}/test-requirements.txt
 commands = find . -type f -name "*.pyc" -delete
            stestr run {posargs}
-Allowlist_externals = find
+allowlist_externals = find
 
 [testenv:pep8]
 commands = flake8