Add PyStructSequence_UnnamedField constant (#5892)

* Add PyStructSequence_UnnamedField constant

* Newsfragment

Add `PyStructSequence_UnnamedField` constant. The string remains the same as of 3.10.

* const

* Change unnamed field to use CString

* Update PyStructSequence_UnnamedField type

Changed type of PyStructSequence_UnnamedField from *const c_char to *const u8.

* Update pyo3-ffi/src/structseq.rs

Co-authored-by: Thomas Tanon <thomas@pellissier-tanon.fr>

* Drop mut

Co-authored-by: David Hewitt <mail@davidhewitt.dev>

* Update newsfragments/5892.added.md

Co-authored-by: David Hewitt <mail@davidhewitt.dev>

* Update pyo3-ffi/src/structseq.rs

---------

Co-authored-by: Thomas Tanon <thomas@pellissier-tanon.fr>
Co-authored-by: David Hewitt <mail@davidhewitt.dev>

Author: 3 people

newsfragments/5892.added.md

@@ -0,0 +1 @@
+Add FFI definition `PyStructSequence_UnnamedField` constant on Python 3.9 and up (or 3.11 with abi3 features).

pyo3-ffi/src/structseq.rs

@@ -19,7 +19,10 @@ pub struct PyStructSequence_Desc {
     pub n_in_sequence: c_int,
 }
 
-// skipped PyStructSequence_UnnamedField;
+extern_libpython! {
+    #[cfg(any(Py_3_11, all(Py_3_9, not(Py_LIMITED_API))))]
+    pub static PyStructSequence_UnnamedField: *const c_char;
+}
 
 extern_libpython! {
     #[cfg(not(Py_LIMITED_API))]