Fix typos in comments (GH-9905)

Modules/gcmodule.c

@@ -504,7 +504,7 @@ move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
                     (void *)young);
             // relink gc_prev to prev element.
             _PyGCHead_SET_PREV(gc, prev);
-            // gc is not COLLECTING state aftere here.
+            // gc is not COLLECTING state after here.
             gc_clear_collecting(gc);
             prev = gc;
         }

Python/ceval.c

@@ -3092,7 +3092,7 @@ main_loop:
                 /* There was an exception and a True return.
                  * We must manually unwind the EXCEPT_HANDLER block
                  * which was created when the exception was caught,
-                 * otherwise the stack will be in an inconsisten state.
+                 * otherwise the stack will be in an inconsistent state.
                  */
                 PyTryBlock *b = PyFrame_BlockPop(f);
                 assert(b->b_type == EXCEPT_HANDLER);

Python/pyarena.c

@@ -49,7 +49,7 @@ struct _arena {
      */
     block *a_head;
 
-    /* Pointer to the block currently used for allocation.  It's
+    /* Pointer to the block currently used for allocation.  Its
        ab_next field should be NULL.  If it is not-null after a
        call to block_alloc(), it means a new block has been allocated
        and a_cur should be reset to point it.
@@ -57,7 +57,7 @@ struct _arena {
     block *a_cur;
 
     /* A Python list object containing references to all the PyObject
-       pointers associated with this area.  They will be DECREFed
+       pointers associated with this arena.  They will be DECREFed
        when the arena is freed.
     */
     PyObject *a_objects;