Update flask-flatpages to 0.6.1

[pyup-bot]

This PR updates Flask-FlatPages from 0.6 to 0.6.1.

The bot wasn't able to find a changelog for this release. Got an idea?

Links

• PyPI: https://pypi.org/project/flask-flatpages
• Repo: https://github.com/SimonSapin/Flask-FlatPages
• Docs: https://pythonhosted.org/Flask-FlatPages/

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