Fix tox4 issues

Use allowlist_externals for rm.

Also, make sure the behaviour is consistent across tox versions
and always skip the installation of the dummy local package
(skip_install for 3, skipsdist does this already for 4).

Change-Id: Iacf9cda958ce410d30a258843e276f902c499e0b

tox.ini

@@ -5,7 +5,7 @@ skipsdist = True
 
 [testenv]
 basepython = python3
-usedevelop = True
+skip_install = True
 setenv =
   VIRTUAL_ENV={envdir}
 deps =
@@ -15,8 +15,10 @@ deps =
 commands = {posargs}
 
 [testenv:docs]
+allowlist_externals =
+  rm
 commands =
-  /bin/rm -fr doc/build/
+  rm -fr doc/build/
   sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:spelling]