chore(deps): update pyo3 dependency to 0.21

[aborgna-q]

This update has a lot of breaking changes, but most of the changes in this PR are just related to replacing rust references with the Bound<'py, T> smart pointer (which keeps track of the python lifetimes).
See migration guide: https://pyo3.rs/v0.21.1/migration.html

Also updated tket-json-rs to 0.4.0, to match the pyo3 dependency.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.86%. Comparing base (8576c49) to head (e04dc56).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #314      +/-   ##
==========================================
+ Coverage   83.92%   84.86%   +0.93%     
==========================================
  Files          31       35       +4     
  Lines        4256     4473     +217     
  Branches     4256     4321      +65     
==========================================
+ Hits         3572     3796     +224     
+ Misses        502      486      -16     
- Partials      182      191       +9     

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