Fix Flask 2.0+ deprecation warning related to using jinja2.Markup, now it uses markupsafe.Markup

[nickjj]

Hi,

I recently updated a project to use Flask 2.0 which came with Jinja 3.0 as a dependency.

After doing that upgrade I started to get this warning from Flask-WTF:

python3.9/site-packages/flask_wtf/form.py:133: DeprecationWarning: 'jinja2.Markup' is deprecated
and will be removed in Jinja 3.1. Import 'markupsafe.Markup' instead.
    return Markup(

This PR takes that advice and imports Markup from markupsafe instead of jinja2.

[nickjj]

The CI failure looks maybe unrelated to this PR, any guidance on how to make it pass?

[greyli]

The CI failure looks maybe unrelated to this PR

It seems related to sphinx-doc/sphinx#9216.

[jugmac00]

I created #437 as a temporary fix for building the docs.

[azmeuk]

Thank you!