www/py-bootstrap-flask: Update to 2.6.0

Upstream now declares the license as an SPDX expression (PEP 639),
which devel/py-setuptools 63.1.0 does not understand yet, so patch
pyproject.toml back to the legacy table form.

Changes: https://github.com/helloflask/bootstrap-flask/blob/2.6.0/CHANGES.rst