Update flask-wtf to 0.15.1

[pyup-bot]

This PR updates Flask-WTF from 0.14.2 to 0.15.1.

Changelog

0.15.1

--------------

Released 2021-05-25

-   Add ``python_requires`` metadata to avoid installing on unsupported
 Python versions. `442`_

.. _442: https://github.com/wtforms/flask-wtf/pull/442

0.15.0

--------------

Released 2021-05-24

-   Drop support for Python < 3.6. `416`_
-   ``FileSize`` validator. `307`_, `365`_
-   Extra requirement ``email`` installs the ``email_validator``
 package. `423`_
-   Fixed Flask 2.0 warnings. `434`_
-   Various documentation fixes. `315`_, `321`_, `335`_, `344`_,
 `386`_, `400`_, `404`_, `420`_, `437`_
-   Various CI fixes. `405`_, `438`_

.. _307: https://github.com/wtforms/flask-wtf/pull/307
.. _315: https://github.com/wtforms/flask-wtf/pull/315
.. _321: https://github.com/wtforms/flask-wtf/pull/321
.. _335: https://github.com/wtforms/flask-wtf/pull/335
.. _344: https://github.com/wtforms/flask-wtf/pull/344
.. _365: https://github.com/wtforms/flask-wtf/pull/365
.. _386: https://github.com/wtforms/flask-wtf/pull/386
.. _400: https://github.com/wtforms/flask-wtf/pull/400
.. _404: https://github.com/wtforms/flask-wtf/pull/404
.. _405: https://github.com/wtforms/flask-wtf/pull/405
.. _416: https://github.com/wtforms/flask-wtf/pull/416
.. _420: https://github.com/wtforms/flask-wtf/pull/420
.. _423: https://github.com/wtforms/flask-wtf/pull/423
.. _434: https://github.com/wtforms/flask-wtf/pull/434
.. _437: https://github.com/wtforms/flask-wtf/pull/437
.. _438: https://github.com/wtforms/flask-wtf/pull/438

0.14.3

--------------

Released 2020-02-06

-   Fix deprecated imports from ``werkzeug`` and ``collections``.

Links

• PyPI: https://pypi.org/project/flask-wtf
• Changelog: https://pyup.io/changelogs/flask-wtf/
• Repo: https://github.com/wtforms/flask-wtf
• Docs: https://pythonhosted.org/Flask-WTF/

[pyup-bot]

Closing this in favor of #636