Python 3.7 support for PyCodeObject. (#2726)

* Python 3.7 support for PyCodeObject.

Co-authored-by: Itamar Turner-Trauring <itamar@pythonspeed.com>

newsfragments/2726.fixed.md

@@ -0,0 +1 @@
+`PyCodeObject` is now once again defined with fields on Python 3.7.

pyo3-ffi/src/cpython/code.rs

@@ -11,9 +11,35 @@ use std::os::raw::{c_char, c_int, c_uchar, c_void};
 #[cfg(all(Py_3_8, not(PyPy), not(Py_3_11)))]
 opaque_struct!(_PyOpcache);
 
-#[cfg(all(not(PyPy), not(Py_3_8)))]
+#[cfg(all(not(PyPy), not(Py_3_7)))]
 opaque_struct!(PyCodeObject);
 
+#[cfg(all(not(PyPy), Py_3_7, not(Py_3_8)))]
+#[repr(C)]
+#[derive(Copy, Clone)]
+pub struct PyCodeObject {
+    pub ob_base: PyObject,
+    pub co_argcount: c_int,
+    pub co_kwonlyargcount: c_int,
+    pub co_nlocals: c_int,
+    pub co_stacksize: c_int,
+    pub co_flags: c_int,
+    pub co_firstlineno: c_int,
+    pub co_code: *mut PyObject,
+    pub co_consts: *mut PyObject,
+    pub co_names: *mut PyObject,
+    pub co_varnames: *mut PyObject,
+    pub co_freevars: *mut PyObject,
+    pub co_cellvars: *mut PyObject,
+    pub co_cell2arg: *mut Py_ssize_t,
+    pub co_filename: *mut PyObject,
+    pub co_name: *mut PyObject,
+    pub co_lnotab: *mut PyObject,
+    pub co_zombieframe: *mut c_void,
+    pub co_weakreflist: *mut PyObject,
+    pub co_extra: *mut c_void,
+}
+
 #[cfg(all(not(PyPy), Py_3_8, not(Py_3_11)))]
 #[repr(C)]
 #[derive(Copy, Clone)]