Switch `once_cell::Lazy` to `LazyLock` when MSRV reaches 1.80

[kylebarron]

I was able to remove almost all uses of once_cell, except for

pyo3-async-runtimes/src/tokio.rs

Line 68 in 5e6c5b7

static TOKIO_BUILDER: Lazy<Mutex<Builder>> = Lazy::new(|| Mutex::new(multi_thread()));

since it looks like LazyLock wasn't added to the stdlib until an MSRV of 1.80.

Originally posted by @kylebarron in #54 (comment)