*: s/whitelist_externals/allowlist_externals/

*: s/whitelist_externals/allowlist_externals/

as allowlist_externals was introduced in
tox v4.0. see
https://github.com/tox-dev/tox/commit/5e33fda1a40ffb4973de3d607a572891eb3cb2d2 , but
this option was backported to 3.18 as an alias of whitelist_externals, so we don't need
to specify the minversion to 4.0 in this change.

as we started using tox 4.0 and up (v4.0.2 in specific). tox complains
and fails like:

alerts-lint: failed with promtool is not allowed, use allowlist_externals to allow it
  alerts-lint: FAIL code 1 (9.25 seconds)

see https://tox.wiki/en/latest/faq.html#tox-4-removed-tox-ini-keys
and https://tox.wiki/en/latest/config.html#allowlist_externals

it'd be nice to use a more inclusive language also. so, in this change,
s/whitelist_externals/allowlist_externals/ in all tox.ini in this
project.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>