Update pyo3 requirement from 0.20 to 0.21

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@v0.20.0...v0.21.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -116,11 +116,11 @@ futures = "0.3"
 inventory = { version = "0.3", optional = true }
 once_cell = "1.14"
 pin-project-lite = "0.2"
-pyo3 = "0.20"
+pyo3 = "0.21"
 pyo3-asyncio-macros = { path = "pyo3-asyncio-macros", version = "=0.20.0", optional = true }
 
 [dev-dependencies]
-pyo3 = { version = "0.20", features = ["macros"] }
+pyo3 = { version = "0.21", features = ["macros"] }
 
 [dependencies.async-std]
 version = "1.12"