Fix Flask 2.3 deprecations

[jkittner]

This fixes all deprecation warnings (url_encode and importing Markup) introduced with Flask 2.3 and continues #562.

I added a changelog entry as requested by the PR template, I hope version 1.1.2 is fine as the new unreleased version?

• fixes flask_wtf no longer compatible with flask 2.3.0 #561
• supersedes Flask 2.3.0 compatibility #562

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change. (tests pass with replace deprecated locked_cached_property decorator with cached_property python-babel/flask-babel#230 and should also pass here once this is merged and released)
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in docs/changes.rst summarizing the change and linking to the issue. Add .. versionchanged:: entries in any relevant code docs.

[azmeuk]

LGTM thank you 🙇