Revisit schemacheck tox env workaround

[phlogistonjohn]

PR #98 added a MULTIDICT_NO_EXTENSIONS=1 env var workaround to get the fedora 39 based ci job working again. The issue is possibly due to:
aio-libs/multidict#887

But I have some doubts about that since Python 3.12 was already in Fedora 39 (I think?).

Regardless, revisit the workaround once the issue above has been resolved and there are python 3.12 wheels available for multidict.

Provisional - Issue assumes PR #98 will be merged.