gh-142447: Fix cast warning in pycore_backoff.h (#142465)

MAKE_VALUE_AND_BACKOFF() macro casts its result to uint16_t.

Add pycore_backoff.h header to test_cppext tests.

Author: vstinner

Include/internal/pycore_backoff.h

@@ -40,7 +40,7 @@ extern "C" {
 #define MAX_VALUE 0x1FFF
 
 #define MAKE_VALUE_AND_BACKOFF(value, backoff) \
-    ((value << BACKOFF_BITS) | backoff)
+    ((uint16_t)(((value) << BACKOFF_BITS) | (backoff)))
 
 // For previous backoff b we use value x such that
 // x + 1 is near to 2**(2*b+1) and x + 1 is prime.

Lib/test/test_cppext/extension.cpp

@@ -14,6 +14,7 @@
 
 #ifdef TEST_INTERNAL_C_API
    // gh-135906: Check for compiler warnings in the internal C API
+#  include "internal/pycore_backoff.h"
 #  include "internal/pycore_frame.h"
 #endif