Fix compilation under Python 3.9+ (MagicStack#610)

Python 3.9 moved a bunch of GC-related symbols around, including `_PyObject_GC_IS_TRACKED` which is used in `recordobj.c`. Fixes: MagicStack#609

Author: elprans

asyncpg/protocol/record/recordobj.c

@@ -8,6 +8,11 @@
 
 #include "recordobj.h"
 
+#ifdef _PyObject_GC_IS_TRACKED
+# define _ApgObject_GC_IS_TRACKED _PyObject_GC_IS_TRACKED
+#else
+# define _ApgObject_GC_IS_TRACKED PyObject_GC_IsTracked
+#endif
 
 static PyObject * record_iter(PyObject *);
 static PyObject * record_new_items_iter(PyObject *);
@@ -57,7 +62,7 @@ ApgRecord_New(PyTypeObject *type, PyObject *desc, Py_ssize_t size)
  return PyErr_NoMemory();
  }
  o = (ApgRecordObject *)type->tp_alloc(type, size);
- if (!_PyObject_GC_IS_TRACKED(o)) {
+ if (!_ApgObject_GC_IS_TRACKED(o)) {
  PyErr_SetString(
  PyExc_TypeError,
  "record subclass is not tracked by GC"