Update pyo3 requirement from =0.20.2 to =0.21.0

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](PyO3/pyo3@v0.20.2...v0.21.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

python/argmin-testfunctions-py/Cargo.toml

@@ -20,4 +20,4 @@ crate-type = ["cdylib"]
 [dependencies]
 argmin_testfunctions = { version = "0.2.0", path = "../../crates/argmin-testfunctions" }
 paste = "1"
-pyo3 = "=0.20.2"
+pyo3 = "=0.21.0"