[2.7] bpo-33767: Fix improper use of SystemError by mmap.mmap objects (GH-7381) (GH-7432)

Raise TypeError instead of SystemError for unsupported operations.
(cherry picked from commit e9e397605789b2a67b67558fbbe756b7b88934f5)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 2ae52c7..bafdce3 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -818,24 +818,6 @@
     }
 }
 
-static PyObject *
-mmap_concat(mmap_object *self, PyObject *bb)
-{
-    CHECK_VALID(NULL);
-    PyErr_SetString(PyExc_SystemError,
-                    "mmaps don't support concatenation");
-    return NULL;
-}
-
-static PyObject *
-mmap_repeat(mmap_object *self, Py_ssize_t n)
-{
-    CHECK_VALID(NULL);
-    PyErr_SetString(PyExc_SystemError,
-                    "mmaps don't support repeat operation");
-    return NULL;
-}
-
 static int
 mmap_ass_slice(mmap_object *self, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
 {
@@ -993,9 +975,9 @@
 
 static PySequenceMethods mmap_as_sequence = {
     (lenfunc)mmap_length,                      /*sq_length*/
-    (binaryfunc)mmap_concat,                   /*sq_concat*/
-    (ssizeargfunc)mmap_repeat,                 /*sq_repeat*/
-    (ssizeargfunc)mmap_item,                           /*sq_item*/
+    0,                                         /*sq_concat*/
+    0,                                         /*sq_repeat*/
+    (ssizeargfunc)mmap_item,                   /*sq_item*/
     (ssizessizeargfunc)mmap_slice,             /*sq_slice*/
     (ssizeobjargproc)mmap_ass_item,            /*sq_ass_item*/
     (ssizessizeobjargproc)mmap_ass_slice,      /*sq_ass_slice*/