rust: update PyO3 dependency

It tripped[^1] on clippy lints on Rustc 1.63:

    error: deref on an immutable reference
     --> rust/src/lib.rs:7:30
      |
    7 | pub fn shannon_entropy(data: &[u8]) -> PyResult<f64> {
      |                              ^ help: if you would like to reborrow, try removing `&*`: `&`
      |
      = note: `-D clippy::borrow-deref-ref` implied by `-D warnings`
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_deref_ref

    error: could not compile `unblob-rust` due to previous error

[^1]: PyO3/pyo3#2555