🚑🧪📝 Allow invoking git in tox

It is called when building the docs. Apparently, `tox -e docs` is not
invoked in CI, neither is it called in RTD, resulting in the
regression having been caught only in local development environments.

This is a follow-up for #12493.

tox.ini

@@ -87,6 +87,8 @@ deps =
     -r{toxinidir}/doc/en/requirements.txt
     # https://github.com/twisted/towncrier/issues/340
     towncrier<21.3.0
+allowlist_externals =
+  git
 commands =
     # Retrieve possibly missing commits:
     -git fetch --unshallow