Pin Flask/Werkzeug 2.3

Avoid Flask 3.0/Werkzeug 3.0 for now as they cause the unit tests to fail:

=================================== FAILURES ===================================
________________________ FunctionalTestCase.test_index _________________________
self = <functional_test.FunctionalTestCase testMethod=test_index>
    def test_index(self):
        client = self.app.test_client()
        response = client.get('/')
        assert response.status_code == 200
        assert response.mimetype == 'text/html'
>       assert response.charset == 'utf-8'
E       AttributeError: 'WrapperTestResponse' object has no attribute 'charset'
client     = <FlaskClient <Flask 'webhook'>>
response   = <WrapperTestResponse streamed [200 OK]>
self       = <functional_test.FunctionalTestCase testMethod=test_index>
tests/functional_test.py:49: AttributeError
----------------------------- Captured stderr call -----------------------------