Changed whitelist to allowlist in tox

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: I4e85c700278fbd68ea38b0c5b036db2c2a72ef76

tox.ini

@@ -31,12 +31,12 @@ commands =
   stestr run {posargs}
   stestr slowest
 
-whitelist_externals =
+allowlist_externals =
   find
 passenv = *_proxy *_PROXY
 
 [testenv:api-ref]
-whitelist_externals = rm
+allowlist_externals = rm
 deps = {[testenv:docs]deps}
 commands =
   rm -rf api-ref/build
@@ -129,7 +129,7 @@ commands =
   rm -fr doc/source/contributor/api/
   rm -fr doc/build/html doc/build/doctrees
   sphinx-build -W -j auto -b html -d doc/build/doctrees doc/source doc/build/html
-whitelist_externals = rm
+allowlist_externals = rm
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
@@ -138,7 +138,7 @@ commands =
   rm -fr doc/build/pdf
   sphinx-build -W -b latex doc/source doc/build/pdf
   make -C doc/build/pdf
-whitelist_externals =
+allowlist_externals =
   make
   rm