Use Py_SET_SIZE() to support Python 3.11

* Replace "Py_TYPE(obj) = type" with "Py_SET_TYPE(obj, type)"
* Replace "Py_SIZE(obj) = size" with "Py_SET_SIZE(obj, size)"
* Replace "Py_REFCNT(obj) = refcnt" with "Py_SET_REFCNT(obj, refcnt)"
* Add pythoncapi_compat.h to get these functions on Python 3.8 and
  older. File copied from:
  https://github.com/pythoncapi/pythoncapi_compat

On Python 3.10, Py_REFCNT() can no longer be used as an l-value to
set a reference count:

* https://docs.python.org/dev/c-api/structures.html#c.Py_REFCNT
* https://docs.python.org/dev/whatsnew/3.10.html#id2

On Python 3.11, Py_TYPE() and Py_SIZE() can no longer as l-values be used to set
an object type and size:

* https://docs.python.org/dev/c-api/structures.html#c.Py_TYPE
* https://docs.python.org/dev/c-api/structures.html#c.Py_SIZE
* https://docs.python.org/dev/whatsnew/3.11.html#id2

src/heapy/heapyc.c

@@ -31,6 +31,7 @@ char heapyc_doc[] =
 #include "compile.h"
 #include "frameobject.h"
 #include "../include/guppy.h"
+#include "../include/pythoncapi_compat.h"
 #include "../sets/nodeset.h"
 #include "hpinit.h"
 #include "heapdef.h"
@@ -224,7 +225,7 @@ INITFUNC (void)
     PyObject *m = NULL;
     PyObject *d;
 
-    Py_TYPE(&_Ny_RootStateStruct) = &NyRootState_Type;
+    Py_SET_TYPE(&_Ny_RootStateStruct, &NyRootState_Type);
 
     // This has to be here because of 'initializer is not a constant'
     // build error on Windows.