Use http error code 422 in CheckView when a merge is not possible. Previously the response used the code 500 in this case. This is not useful because it's not an internal server error when the check whether a PDF file can be merged fails because that's the point of the CheckView. The code used now means "Unprocessable Entity" and fits better. Part of mpuls/issue6009

flask>=0.7
requests
py3o.template>=0.8
py3o.renderers.pyuno>=0.3
setuptools
pypdf2
six>=1.9