Convert more of src/x509/ocsp_req.rs to new pyo3 APIs

[facutuesca]

Part of #10676

This fixes all except one of the migration warnings for src/x509/ocsp_req.rs. The remaining warning is:

error[E0277]: the trait bound `(&[u8], &[u8], pyo3::Bound<'_, pyo3::types::PyLong>, &pyo3::PyAny): pyo3::FromPyObject<'_>` is not satisfied
    --> src/x509/ocsp_req.rs:199:14
     |
199  |             .extract()?;
     |              ^^^^^^^ the trait `pyo3::FromPyObject<'_>` is not implemented for `(&[u8], &[u8], pyo3::Bound<'_, pyo3::types::PyLong>, &pyo3::PyAny)`, which is required by `(&[u8], &[u8], pyo3::Bound<'_, pyo3::types::PyLong>, &pyo3::PyAny): pyo3::conversion::FromPyObjectBound<'_, '_>`
     |

cc @alex @reaperhulk