* gnu/packages/mail.scm (khard): Update to 0.20.0.
[build-system]: Switch to pyproject-build-system.
[arguments]: Add test-flags to skip some tests.
[inputs]: Remove python-atomicwrites and python-unidecode.
[native-inputs]: Add python-pytest and python-setuptools.
Change-Id: I95a70042a8f153d7a983db49e6acd8430718fa71