Convert more of src/pkcs7.rs to new pyo3 APIs

[facutuesca]

Part of #10676

This fixes all except one of the migration warnings for src/pkcs7.rs. The remaining warning is:

error: use of deprecated associated function `pyo3::types::PyBytes::new`: `PyBytes::new` will be replaced by `PyBytes::new_bound` in a future PyO3 version
   --> src/pkcs7.rs:168:54
    |
168 |             let digest_bytes = pyo3::types::PyBytes::new(py, &digest);
    |       

which, when fixed, causes the following compilation error:

error[E0597]: `digest_bytes` does not live long enough
   --> src/pkcs7.rs:172:40
    |
168 |             let digest_bytes = pyo3::types::PyBytes::new_bound(py, &digest);
    |                 ------------ binding `digest_bytes` declared here
...
172 |                     asn1::parse_single(digest_bytes.as_bytes()).unwrap(),
    |                                        ^^^^^^^^^^^^ borrowed value does not live long enough
...
194 |         };
    |         - `digest_bytes` dropped here while still borrowed

cc @alex @reaperhulk

[facutuesca]

Another GHA error:

.nox/tests/lib/python3.12/site-packages/_pytest/doctest.py:426: in collect
    import doctest
E   SyntaxError: source code string cannot contain null bytes