Keyword only arguments

[dvermd]

Thank you for contributing to pyo3!

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 or make test if you need to test examples)
• Rust lints (make clippy)
• Rust formatting (cargo fmt)
• Python formatting (black . --check. You can install black with pip install black)
• Compatibility with all supported Python versions for all examples. This uses tox; you can do run it using make test_py.

You can run a similar set of checks as the CI pipeline using make test.

Closes #1025

[kngwyu]

LGTM, thank you!