Switch to allowlist_externals for modern Tox (#4574)

`whitelist_externals` is now deprecated in Tox >= 3.18 and has been renamed to `allowlist_externals`

https://tox.readthedocs.io/en/latest/config.html#conf-allowlist_externals

docs/faq.md

@@ -65,7 +65,7 @@ isolated_build = true
 envlist = py27, py36
 
 [testenv]
-whitelist_externals = poetry
+allowlist_externals = poetry
 commands =
     poetry install -v
     poetry run pytest tests/

pyproject.toml

@@ -58,7 +58,7 @@ pytest = "^6.2"
 pytest-cov = "^2.8"
 pytest-mock = "^3.5"
 pre-commit = { version = "^2.6", python = "^3.6.1" }
-tox = "^3.0"
+tox = "^3.18"
 pytest-sugar = "^0.9"
 httpretty = "^1.0"
 zipp = { version = "^3.4", python = "<3.8"}

tox.ini

 [tox]
-minversion = 3.3.0
+minversion = 3.18.0
 isolated_build = True
 envlist = py36, py37, py38, py39
 
 [testenv]
-whitelist_externals = poetry
+allowlist_externals = poetry
 commands =
     poetry install -vv --no-root
     poetry run pytest {posargs} tests/