Update flask-bootstrap to 3.3.7.1

[pyup-bot]

There's a new version of Flask-Bootstrap available.
You are currently using 3.3.5.6. I have updated it to 3.3.7.1

These links might come in handy: PyPI | Changelog | Repo | Docs

Changelog

3.3.7.0

---

• Most likely to be the last major Bootstrap 3 release (see
  Getting to v4 twbs/bootstrap#20631). Make sure you pin
  Flask-Bootstrap's version to <4 to avoid inadvertent updates.

3.3.5.7

---

• IE-specific meta information and viewport meta tags have been added to the
  base template.
• The flashed messages are no longer considered "safe", which means HTML-markup
  will be escaped. See :func:flashed_messages for more information.
• The title block now contains an empty-by-default title variable.

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖