ffi: add missing definition PyCMethod_New

CHANGELOG.md

@@ -91,6 +91,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Use the Rust function path for `wrap_pymodule!` of a `#[pymodule]` with a `#[pyo3(name = "..")]` attribute, not the Python name. [#2081](https://github.com/PyO3/pyo3/pull/2081)
 - Fix panic in `#[pyfunction]` generated code when a required argument following an `Option` was not provided.  [#2093](https://github.com/PyO3/pyo3/pull/2093)
 - Fixed undefined behaviour caused by incorrect `ExactSizeIterator` implementations. [#2124](https://github.com/PyO3/pyo3/pull/2124)
+- Fix missing FFI definition `PyCMethod_New` on Python 3.9 and up. [#2143](https://github.com/PyO3/pyo3/pull/2143)
 - Add missing FFI definitions `_PyLong_NumBits` and `_PyLong_AsByteArray` on PyPy. [#2146](https://github.com/PyO3/pyo3/pull/2146)
 - Fix memory leak in implementation of `AsPyPointer` for `Option<T>`. [#2160](https://github.com/PyO3/pyo3/pull/2160)
 - Fix the signature of `_PyLong_NumBits` [#2161](https://github.com/PyO3/pyo3/pull/2161)

pyo3-ffi/src/methodobject.rs

@@ -117,6 +117,7 @@ pub union PyMethodDefPointer {
 const _: () =
     [()][mem::size_of::<PyMethodDefPointer>() - mem::size_of::<Option<extern "C" fn()>>()];
 
+#[cfg(not(Py_3_9))]
 extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyCFunction_New")]
     pub fn PyCFunction_New(ml: *mut PyMethodDef, slf: *mut PyObject) -> *mut PyObject;
@@ -129,7 +130,32 @@ extern "C" {
     ) -> *mut PyObject;
 }
 
-// skipped non-limited / 3.9 PyCMethod_New
+#[cfg(Py_3_9)]
+#[inline]
+pub unsafe fn PyCFunction_New(ml: *mut PyMethodDef, slf: *mut PyObject) -> *mut PyObject {
+    PyCFunction_NewEx(ml, slf, std::ptr::null_mut())
+}
+
+#[cfg(Py_3_9)]
+#[inline]
+pub unsafe fn PyCFunction_NewEx(
+    ml: *mut PyMethodDef,
+    slf: *mut PyObject,
+    module: *mut PyObject,
+) -> *mut PyObject {
+    PyCMethod_New(ml, slf, module, std::ptr::null_mut())
+}
+
+#[cfg(Py_3_9)]
+extern "C" {
+    #[cfg_attr(PyPy, link_name = "PyPyCMethod_New")]
+    pub fn PyCMethod_New(
+        ml: *mut PyMethodDef,
+        slf: *mut PyObject,
+        module: *mut PyObject,
+        cls: *mut PyTypeObject,
+    ) -> *mut PyObject;
+}
 
 /* Flag passed to newmethodobject */
 pub const METH_VARARGS: c_int = 0x0001;