[2.7] Fix misleading mentions of tp_size in comments (GH-9138)

Many type object initializations labeled a field "tp_size" in the
comment, but the name of that field is tp_basicsize..
(cherry picked from commit 0e0bc4e221f592f305d335faf5f8046484eb9238)

Co-authored-by: Peter Eisentraut <peter@eisentraut.org>
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index bafdce3..02b31ca 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -1026,7 +1026,7 @@
 static PyTypeObject mmap_object_type = {
     PyVarObject_HEAD_INIT(NULL, 0)
     "mmap.mmap",                                /* tp_name */
-    sizeof(mmap_object),                        /* tp_size */
+    sizeof(mmap_object),                        /* tp_basicsize */
     0,                                          /* tp_itemsize */
     /* methods */
     (destructor) mmap_object_dealloc,           /* tp_dealloc */