Update pyo3 requirement from 0.19.0 to 0.20.0 #414

Job	Run time
Check	17s
Lints	4m 5s
Test Suite (ubuntu-latest, MSRV)	2m 53s
no-std build (aarch64-unknown-none)	21s
Test Suite (ubuntu-latest, stable)	2m 41s
no-std build (thumbv6m-none-eabi)	21s
Test Suite (macos-latest, MSRV)	3m 32s
Test Suite (macos-latest, stable)	3m 0s
Coverage	39s
Test Suite (windows-latest, MSRV)	7m 36s
Test Suite (windows-latest, stable)	7m 10s
Release	0s
	32m 35s

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