bpo-31018: Switch to #pragma pack from __declspec(align) (#2848)

Modules/_tracemalloc.c

@@ -83,7 +83,7 @@ typedef struct
 #ifdef __GNUC__
 __attribute__((packed))
 #elif defined(_MSC_VER)
-_declspec(align(4))
+#pragma pack(push, 4)
 #endif
 {
     /* filename cannot be NULL: "<unknown>" is used if the Python frame
@@ -91,6 +91,9 @@ _declspec(align(4))
     PyObject *filename;
     unsigned int lineno;
 } frame_t;
+#ifdef _MSC_VER
+#pragma pack(pop)
+#endif
 
 
 typedef struct {