Adhere to tox INI rules

refs:
* tox-dev/tox@f4840a0
* https://tox.wiki/en/latest/faq.html#tox-4-changed-ini-rules
* tox-dev/tox#2676

Signed-off-by: Francesco Giordano <giordafr@amazon.it>

Author: francesco-giordano

tox.ini

@@ -5,7 +5,9 @@ envlist =
 
 # Default testenv. Used to run tests on all python versions.
 [testenv]
-passenv = CI GITHUB_*
+passenv =
+    CI
+    GITHUB_*
 usedevelop =
     cov: true
     nocov: false