Keep PyMethodDef alive through a weakref in Python 3.9

Author: YannickJadoul

include/pybind11/pybind11.h

@@ -369,6 +369,15 @@ class cpp_function : public function {
             }
 
             m_ptr = PyCFunction_NewEx(rec->def, rec_capsule.ptr(), scope_module.ptr());
+
+#if PY_VERSION_HEX >= 0x03090000
+            weakref(m_ptr, cpp_function([rec_capsule](handle wr) {
+                rec_capsule.dec_ref();
+                wr.dec_ref();
+            })).release();
+            rec_capsule.inc_ref();
+#endif
+
             if (!m_ptr)
                 pybind11_fail("cpp_function::cpp_function(): Could not allocate function object");
         } else {
@@ -466,7 +475,7 @@ class cpp_function : public function {
             }
             if (rec->def) {
                 std::free(const_cast<char *>(rec->def->ml_doc));
-                //delete rec->def;
+                delete rec->def;
             }
             delete rec;
             rec = next;