www/py-flask: Clean up BUILD_DEPENDS

from pyproject.toml:
[build-system]
requires = ["flit_core<4"]
build-backend = "flit_core.buildapi"

from CHANGES.rst:

• Use flit_core instead of setuptools as build backend.

Reference: https://github.com/pallets/flask/blob/3.0.0/pyproject.toml#L46-L48

		https://github.com/pallets/flask/blob/3.0.0/CHANGES.rst#version-233

Approved by: portmgr (blanket)
With hat: python