🚑🧪📝 Stop capping Towncrier @ local docs builds

It is uncapped in RTD. 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

@@ -85,8 +85,6 @@ basepython = python3
 usedevelop = True
 deps =
     -r{toxinidir}/doc/en/requirements.txt
-    # https://github.com/twisted/towncrier/issues/340
-    towncrier<21.3.0
 allowlist_externals =
   git
 commands =