Bump tox minversion to 3.18.0

Since tox 3.18.0, the whitelist_externals option has been deprecated in
favor of the new allow_list_externals option[1].

This change bumps the minversion of tox so that we can replace
the deprecated option.

This also adds requirements.txt to the docs tox environment, to fix
the openstack-tox-docs job.

[1] https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Co-Authored-By: Grzegorz Grasza <xek@redhat.com>
Change-Id: I9066385091c80405c265bf95728c2011a8ad4780

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py38,py39,pep8
 skipsdist = True
 ignore_basepython_conflict = True
@@ -24,7 +24,7 @@ commands =
     coverage html -d cover
     coverage xml -o cover/coverage.xml
     coverage report -m
-whitelist_externals = rm
+allowlist_externals = rm
 
 [testenv:debug]
 commands = oslo_debug_helper -t barbicanclient/tests {posargs}
@@ -40,15 +40,17 @@ commands = {posargs}
 # the main docs to https://docs.openstack.org/python-barbicanclient
 description = Build main documentation
 basepython = python3
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+       -r{toxinidir}/requirements.txt
+       -r{toxinidir}/doc/requirements.txt
 commands=
     rm -rf doc/build/html doc/build/doctrees
     sphinx-build -W -b html -d doc/build/doctrees doc/source doc/build/html
-whitelist_externals = rm
+allowlist_externals = rm
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
   rm
 commands =