Compile warning in _testcapi/dict.c on macos

[sobolevn]

Bug report

When you compile CPython on macos, you get:

gcc  -fno-strict-overflow -Wsign-compare -g -Og -Wall -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include  -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal  -I. -I./Include -I/opt/homebrew/opt/openssl/include -I/opt/homebrew/opt/openssl/include   -c ./Modules/_testcapi/dict.c -o Modules/_testcapi/dict.o
./Modules/_testcapi/dict.c:289:16: warning: result of comparison against a string literal is unspecified (use an explicit string comparison function instead) [-Wstring-compare]
    assert(key == UNINITIALIZED_PTR);
               ^  ~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/assert.h:99:25: note: expanded from macro 'assert'
    (__builtin_expect(!(e), 0) ? __assert_rtn(__func__, __ASSERT_FILE_NAME, __LINE__, #e) : (void)0)
                        ^
./Modules/_testcapi/dict.c:290:18: warning: result of comparison against a string literal is unspecified (use an explicit string comparison function instead) [-Wstring-compare]
    assert(value == UNINITIALIZED_PTR);
                 ^  ~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/assert.h:99:25: note: expanded from macro 'assert'
    (__builtin_expect(!(e), 0) ? __assert_rtn(__func__, __ASSERT_FILE_NAME, __LINE__, #e) : (void)0)
                        ^
2 warnings generated.

It is defined as:

/* Marker to check that pointer value was set. */
#define UNINITIALIZED_PTR ((void *)"uninitialized")

[sobolevn]

Duplicate of #109469