migration to pyo3 0.21 new Bound API

[Icxolu]

pyo3 0.21 is on the final stretch to release, with an initial beta expected in the next few days.

This starts the migration to pyo3s new Bound API. This initial PR bumps the pyo3 version, enables the gil-refs feature (to suppress most of the deprecation warnings) and adjust the rest to compile and pass the tests again.

Note:
PyTypeInfo needs to be updated to is_type_of_bound now, because otherwise pyo3s internals will not use the custom implementation, as found in PyO3/pyo3#3929

I've prepared followups migrating different parts of the API in (hopefully) reviewable chunks. Generally I took a similar approach to pyo3 itself, leaving old API around (for easy migrations) and adding deprecation warnings where necessary.

I'll leave this as a draft until the beta release drops and rebase this than.

[Icxolu]

Rebased to use pyo3 0.21.0-beta, bumped examples to also use it and fixed MSRV

[adamreichold]

Thank for bringing rust-numpy up to speed. Hope we'll be able to work through supporting the Bound API and NumPy 2 at the same time without too much toe stepping.