Python: build against abi3 to support multiple Python versions

- Upgrade to pyo3 0.21 to enable timedelta for chrono
- Enable abi3 for multi-python support

api/python/lib.rs

@@ -23,7 +23,7 @@ fn quit_event_loop() -> Result<(), errors::PyEventLoopError> {
 use pyo3::prelude::*;
 
 #[pymodule]
-fn slint(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
+fn slint(_py: Python<'_>, m: &Bound<'_, PyModule>) -> PyResult<()> {
     i_slint_backend_selector::with_platform(|_b| {
         // Nothing to do, just make sure a backend was created
         Ok(())