fix: add tox 4 support in tox.ini

fix: add tox 4 support in tox.ini

Tox 4 introduced configuration syntax changes to tox.ini that requires
at least tox 3.18 for compatibility.

The 'passenv' configuration was documented as space separated in tox 3
and is now comma separated in tox 4. However, newline separated works in
both tox versions [1].

The 'whitelist_externals' configuration was replaced by
'allowlist_externals', the new value was introduced in 3.18 as an
alternative.

[1] https://tox.wiki/en/latest/upgrading.html#upgrading

Change-Id: Ibe6d16f7a180d4341f1a2c0f4a6343d72210cd94
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>