Support tox4

Renamed deprecated option whitelist_externals to allowlist_externals.

Bumped tox minversion as allowlist_externals is only supported in
3.18 [1].

Also set skipsdist=False (default) due to [2].

[1] https://tox.wiki/en/3.24.5/config.html#conf-allowlist_externals
[2] https://github.com/tox-dev/tox/issues/2730

Change-Id: Iee2f00418804c21589bd8abc19a8b25aff458e77