Use allowlist_externals, instead of whitelist_externals

[ni-jfitzger]

• This contribution adheres to CONTRIBUTING.md.
• [ ] I've updated CHANGELOG.md if applicable.
• [ ] I've added tests applicable for this pull request

What does this Pull Request accomplish?

Use "allowlist_externals", instead of "whitelist_externals" in our tox inis.

"whitelist_externals" externals was deprecated in tox 3.18 in favor of "allowlist_externals" which works the exact same way. "whitelist_externals" is no longer supported in tox 4.

Notes:

• It turns out we're actually installing tox 4.4 in our nimibot github-runners.
  • If we were doing more than just running our system tests, we would probably be failing without this change.
• travis-ci is still using tox 3.x and can't upgrade to 4.x due to incompatibility with tox-travis. See Upgrade tox dependency when tox-travis becomes compatible with tox 4.0 and later #1876. However, it's using tox 3.28 which supports allowlist_externals.
• My local WSL installation has tox 4.4 installed natively and I had to do a special installation of tox 3. In order to use that one instead of tox 4, I have to use sudo. Entering a password everytime I run tox is driving me crazy.

List issues fixed by this Pull Request below, if any.

What testing has been done?

None. If travis-ci and the other checks pass, then it's good.