testing/py3-marshmallow: upgrade to 3.19.0
https://github.com/marshmallow-code/marshmallow/blob/3.19.0/CHANGELOG.rst
funnily enough, changelog for unreleased 3.20 says "Officially support Python 3.11", which.. just adds it to the pypi metadata
https://github.com/marshmallow-code/marshmallow/blob/3.19.0/CHANGELOG.rst
funnily enough, changelog for unreleased 3.20 says "Officially support Python 3.11", which.. just adds it to the pypi metadata