Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 in order to
replace tox's whitelist_externals by allowlist_externals option:
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: I244d98bb3fc7cb75624b598f4d26f784159f5428

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist = py27,py3,pep8
-minversion = 2.0
+minversion = 3.18.0
 skipsdist = True
 
 [testenv]
@@ -17,7 +17,7 @@ commands = sh -c '(find . -not \( -type d -name .?\* -prune \) \
            \( -type d -name "__pycache__" -or -type f -name "*.py[co]" \) \
            -print0) | xargs -0 rm -rf'
            stestr run {posargs}
-whitelist_externals = sh
+allowlist_externals = sh
 passenv = SWIFT_* *_proxy
 
 [testenv:pep8]
@@ -45,7 +45,7 @@ basepython = python3
 setenv =
     OS_TEST_PATH=test.functional
     PYTHON=coverage run --source swiftclient --parallel-mode
-whitelist_externals =
+allowlist_externals =
     coverage
     rm
 commands =
@@ -59,7 +59,7 @@ commands =
 [testenv:py2func]
 basepython=python2
 setenv = {[testenv:func]setenv}
-whitelist_externals = {[testenv:func]whitelist_externals}
+allowlist_externals = {[testenv:func]allowlist_externals}
 commands = {[testenv:func]commands}
 
 [testenv:docs]
@@ -107,7 +107,7 @@ commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasen
 [testenv:pdf-docs]
 basepython = python3
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf