Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 python 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: Id018ab220cc95f39aea9cbce6880ba65f7f715f5

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist = py36,py37,pep8,npm-test,npm-lint
-minversion = 3.1.1
+minversion = 3.18.0
 skipsdist = True
 ignore_basepython_conflict = True
 
@@ -8,7 +8,7 @@ ignore_basepython_conflict = True
 basepython = python3
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
-whitelist_externals = *
+allowlist_externals = *
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
@@ -23,11 +23,11 @@ deps = -r{toxinidir}/doc/requirements.txt
 commands =
    rm -rf doc/build
    sphinx-build -W -b html doc/source doc/build/html
-whitelist_externals = rm
+allowlist_externals = rm
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
    make
 commands =
    sphinx-build -W -b latex doc/source doc/build/pdf