web: use html.unescape() when available (Python 3.4+)

[dgw]

Description

Fixes #2203 on Python 3.4+ hosts. Anything older, well, we're not going to ship a reimplementation of Python's html.unescape() function (and everything else that actually makes it work) in a patch release when we've already dropped support for those Python versions on master. This bug just isn't a showstopper. It's not worth that.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
  • Well, locally make quality raises E741 on a variable name in sopel/modules/help.py, but that's nothing to do with this patch and it's not going to be an issue on the older version of flake8 configured for use on 7.1.x.
• I have tested the functionality of the things this change touches
  • Proof that the kind of link in "You[apos]re" result from Sopel's URL parser #2203 works now:

    <dgw> https://matrix.to/#/#element-community-testing:matrix.org
    <SopelTest> [url] You're invited to talk on Matrix | matrix.to