migration to pyo3 0.21 new `Bound` API #808

Job	Run time
examples	1m 42s
check-msrv	4m 7s
lint	1m 7s
valgrind	0s
python${{ matrix.python-version }}-${{ matrix.platform.python-architecture }} ${{ matrix.platform.os }}	0s
coverage	0s
careful	0s
cross-build	0s
	6m 56s

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