[amd][driver] fix _PyFloat_Pack2 for Python 3.10 (#7879)

Fixing Python 3.10 build error on AMD:

```
E              95 |    if (code != HIP_SUCCESS)
E                 |        ~~~~ ^  ~~~~~~~~~~~
E           __triton_launcher.c:172:5: error: no matching function for call to '_PyFloat_Pack2'
E             172 |     _PyFloat_Pack2(f, (char*)&result, 1);
E                 |     ^~~~~~~~~~~~~~
E           note: candidate function not viable: no known conversion from 'char *' to 'unsigned char *' for 2nd argument
E              87 | PyAPI_FUNC(int) _PyFloat_Pack2(double x, unsigned char *p, int le);
```

Author: yuguo68

third_party/amd/backend/driver.py

@@ -474,9 +474,9 @@ def format_of(ty):
 
 static uint16_t pack_fp16(double f) {{
     uint16_t result;
-    // from https://github.com/python/pythoncapi-compat
+    // from https://github.com/python/pythoncapi-compat/blob/5e317108f872c904eb726cb8d560dcadbdf88a72/pythoncapi_compat.h#L482-L492
 #if 0x030600B1 <= PY_VERSION_HEX && PY_VERSION_HEX <= 0x030B00A1 && !defined(PYPY_VERSION)
-    _PyFloat_Pack2(f, (char*)&result, 1);
+    _PyFloat_Pack2(f, (unsigned char*)&result, 1);
 #else
     PyFloat_Pack2(f, (char*)&result, 1);
 #endif