Update import of markupsafe.Markup

[cthoyt]

This addresses a deprecation warning for the upcoming minor release of Flask:

DeprecationWarning: 'flask.Markup' is deprecated and will be removed in Flask 2.4. Import 'markupsafe.Markup' instead.
    from flask import current_app, Markup, Blueprint, url_for

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (22374ff) 100.00% compared to head (de3d792) 100.00%.

:mega: This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #293 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 1 1 Lines 135 136 +1 Branches 16 16 ========================================= + Hits 135 136 +1 ``` | [Impacted Files](https://codecov.io/gh/helloflask/bootstrap-flask/pull/293?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=helloflask) | Coverage Δ | | |---|---|---| | [flask\_bootstrap/\_\_init\_\_.py](https://codecov.io/gh/helloflask/bootstrap-flask/pull/293?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=helloflask#diff-Zmxhc2tfYm9vdHN0cmFwL19faW5pdF9fLnB5) | `100.00% <100.00%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[greyli]

Hi, thanks for working on this! I'm going to merge #296 which is a more complete fix for this issue.

[cthoyt]

thanks @greyli