Update rust backend to pyo3 0.22.6

[ngoldbaum]

See https://pyo3.rs/v0.23.4/migration.html#from-021-to-022 for more details. This passes the tests using the rust backend on my local Mac setup except for one test that is also failing on CI.

I also tried updating to 0.23 as well but the update to PyO3 0.23 is a little trickier. Here are the compiler errors I see: https://gist.github.com/ngoldbaum/467cebc15f9ec79b8930c389ddfea115

Most of the errors are similar to other PyO3 0.23 migrations I've done, but you probably want to think about how to safely add unsafe impl Sync implementations for the various types that are flagged under 0.23 because they're not Sync.