Update Rust crate pyo3 to 0.20.3

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
pyo3	dependencies	patch	0.20.2 -> 0.20.3

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Release Notes

pyo3/pyo3 (pyo3)

v0.20.3

Compare Source

Packaging

• Add portable-atomic dependency. #​3619
• Check maximum version of Python at build time and for versions not yet supported require opt-in to the abi3 stable ABI by the environment variable PYO3_USE_ABI3_FORWARD_COMPATIBILITY=1. #​3821

Fixed

• Use portable-atomic to support platforms without 64-bit atomics. #​3619
• Fix compilation failure with either feature enabled without experimental-inspect enabled. #​3834

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