Prep for 0.28 release

[kylebarron]

Thank you for contributing to pyo3-async-runtimes!

Please consider adding the following to your pull request:

• an entry in CHANGELOG.md
• docs to all new functions and / or detail in the guide
• tests for all new or changed functions

Be aware the CI pipeline will check your pull request for the following:

• Rust tests (Just cargo test)
• Rust lints (make clippy)
• Rust formatting (cargo fmt)
• Python formatting (black . --check. You can install black with pip install black)