frozen pyclasses (#972)

### Change list

- Add the
[`frozen`](https://pyo3.rs/v0.23.3/class.html?highlight=frozen#frozen-classes-opting-out-of-interior-mutability)
attribute to `#[pyclass]`. This should be a small performance
improvement, and allows for accessing the interior of a class even
without the GIL.

Ref developmentseed/obstore#118