Bumped Pyo3 to 0.20

[remusao]

I will have a look into the failing tests.

[remusao]

I have updated the PR with more changes so that tests are green (at least locally).

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (cfce252) 80.99% compared to head (3d8678a) 81.05%. Report is 7 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #44 +/- ## ========================================== + Coverage 80.99% 81.05% +0.05% ========================================== Files 4 3 -1 Lines 1021 1024 +3 ========================================== + Hits 827 830 +3 Misses 194 194 ``` | [Files](https://app.codecov.io/gh/davidhewitt/pythonize/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Hewitt) | Coverage Δ | | |---|---|---| | [src/ser.rs](https://app.codecov.io/gh/davidhewitt/pythonize/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Hewitt#diff-c3JjL3Nlci5ycw==) | `86.25% <100.00%> (+0.14%)` | :arrow_up: | | [src/de.rs](https://app.codecov.io/gh/davidhewitt/pythonize/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Hewitt#diff-c3JjL2RlLnJz) | `88.43% <92.30%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/davidhewitt/pythonize/pull/44/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Hewitt)

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[Vonfry]

What is the plan for the next release?

[davidhewitt]

This has been released in pythonize 0.20