Update pyo3 requirement from 0.19.0 to 0.20.0 #410

Job	Run time
Check	16s
Lints	3m 55s
no-std build (aarch64-unknown-none)	24s
Test Suite (ubuntu-latest, MSRV)	2m 27s
no-std build (thumbv6m-none-eabi)	22s
Test Suite (ubuntu-latest, stable)	2m 10s
Test Suite (macos-latest, MSRV)	1s
Test Suite (macos-latest, stable)	2m 27s
Test Suite (windows-latest, MSRV)	2m 13s
Test Suite (windows-latest, stable)	1m 48s
Coverage	46s
Release	0s
	16m 49s

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