Add `abi3` feature to pyo3. #54

Job	Run time
linux (x86_64)	1m 13s
macos (x86_64)	1m 6s
linux (x86)	1m 31s
sdist	20s
linux (aarch64)	1m 47s
linux (armv7)	2m 25s
macos (aarch64)	50s
linux (s390x)	1m 43s
linux (ppc64le)	1m 44s
windows (x64)	2m 22s
windows (x86)	3m 39s
Check clippy for Python bindings	41s
Release	0s
	19m 21s

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