add reno to allowlist_externals

enables adding a release note with tox 4

Change-Id: Ia32da2b511b1cd901d74c6b2f126d0c0bc910dff

doc/source/contributor/adding_release_notes.rst

@@ -70,7 +70,7 @@ environment in manila:
 
 .. code-block:: console
 
-    $ tox -e newnote slug-goes-here
+    $ tox -e newnote -- slug-goes-here
 
 .. note::
 

tox.ini

@@ -33,6 +33,7 @@ commands =
 allowlist_externals = rm
 
 [testenv:newnote]
+allowlist_externals = reno
 deps =
 -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
@@ -115,7 +116,7 @@ usedevelop = False
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source manila --parallel-mode
-allowlist_externals = 
+allowlist_externals =
   {toxinidir}/tools/cover.sh
 commands =
   {toxinidir}/tools/cover.sh {posargs}