Update pyo3 requirement from 0.20.2 to 0.21.0 (#190)

Updates the requirements on [pyo3](https://github.com/pyo3/pyo3) to permit the latest version.
- [Release notes](https://github.com/pyo3/pyo3/releases)
- [Changelog](https://github.com/PyO3/pyo3/blob/main/CHANGELOG.md)
- [Commits](https://github.com/pyo3/pyo3/compare/v0.20.2...v0.21.0)

---
updated-dependencies:
- dependency-name: pyo3
  dependency-type: direct:production
...

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bindings/python/Cargo.toml

@@ -14,7 +14,7 @@ crate-type = ["cdylib"]
 [dependencies]
 anyhow = "1.0.79"
 ordered-float = "4.2.0"
-pyo3 = {version = "0.20.2", features = ["anyhow", "extension-module"] }
+pyo3 = {version = "0.21.0", features = ["anyhow", "extension-module"] }
 regorus = { path  = "../.." }
 serde_json = "1.0.112"