gh-100519: simplification to `eff_request_host` in cookiejar.py (#99588)

`IPV4_RE` includes a `.`, and the `.find(".") == -1` included here is already testing to make sure there's no dot, so this part of the expression is tautological. Instead use more modern `in` syntax to make it clear what the check is doing here. The simplified implementation more clearly matches the wording in RFC 2965.

Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>