Add argument `none_value` for `None` representation in loading and dumping #318

Job	Run time
lint	38s
test py3.8	1m 15s
test py3.9	57s
build os=${{ matrix.os }} target=${{ matrix.target }} int=${{ matrix.interpreter \|\| 'all' }} ml=${{ matrix.manylinux \|\| 'auto' }}	0s
test py3.10	45s
test py3.11	47s
test py3.12	53s
inspect-pypi-assets	0s
check	8s
release	0s
	5m 23s

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