Address flask.Markup deprecation in Flask 2.3

[guidoiaquinti]

This PR fixes the flask.Markup deprecation in Flask 2.3 (see upstream changelog)

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (22374ff) 100.00% compared to head (f7d1481) 100.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #296 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 1 1 Lines 135 136 +1 Branches 16 16 ========================================= + Hits 135 136 +1 ``` | [Impacted Files](https://app.codecov.io/gh/helloflask/bootstrap-flask/pull/296?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://app.codecov.io/gh/helloflask/bootstrap-flask/pull/296?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)` | |

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[greyli]

Merged, thanks!