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
The patch also sets ubuntu-focal nodeset for the docs job as newer
ubuntu (with newer tox available) is required.

Change-Id: I5b32e7f65710199601a6856dba86584390eb92b7

.zuul.yaml

@@ -1,4 +1,15 @@
 - project:
     templates:
-      - nodejs4-jobs
-      - nodejs10-docs
+      - nodejs14-jobs
+    check:
+      jobs:
+        - nodejs-run-docs:
+            vars:
+              node_version: 14
+            nodeset: ubuntu-focal
+    gate:
+      jobs:
+        - nodejs-run-docs:
+            vars:
+              node_version: 14
+            nodeset: ubuntu-focal

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.4.0
+minversion = 3.18.0
 skipsdist = True
 ignore_basepython_conflict = True
 
@@ -20,11 +20,11 @@ deps =
 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