Convert private_bytes methods to new pyo3 APIs

[facutuesca]

Part of #10676

Converts pkey_private_bytes() in utils.rs, along with all of the private_bytes() methods using it.
There's also an extra migration for rsa.rs, which needs manual review:

// rsa.rs:150
// before
.extract::<Option<&[u8]>>()?
// after
.extract::<Option<pyo3::pybacked::PyBackedBytes>>()?

cc @alex @reaperhulk

[alex]

Some failing tests, ptal

[facutuesca]

Some failing tests, ptal

I can't reproduce locally, and the GH Action logs only show:

I'll trigger another run to see if anything changes

[alex]

Oh, whoops. (Working from phone)

…

On Thu, Apr 4, 2024, 10:17 AM Facundo Tuesca ***@***.***> wrote: Some failing tests, ptal I can't reproduce locally, and the GH Action logs only show: image.png (view on web) <https://github.com/pyca/cryptography/assets/5762120/ccabd306-573c-4eee-88c7-c43c21799c36> I'll trigger another run to see if anything changes — Reply to this email directly, view it on GitHub <#10707 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAAGBFWSXO6SQ6LDPE3YQ3Y3VOHBAVCNFSM6AAAAABFXKAQBCVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDAMZXGMZTONJYHA> . You are receiving this because you were mentioned.Message ID: ***@***.***>