fix(.pyi): xxh64 is a separate C type, not an alias for xxh3_64

xxh64 uses the classic XXH64 algorithm (XXH64_state_t) while xxh3_64
uses the XXH3 algorithm. The .pyi incorrectly declared them as type
aliases, lying to type checkers. xxh128 = xxh3_128 remains correct,
that's a real Python-level alias.

Author: ifduyue

xxhash/__init__.pyi

@@ -64,13 +64,15 @@ class _Hasher:
 @final
 class xxh32(_Hasher): ...
 
+@final
+class xxh64(_Hasher): ...
+
 @final
 class xxh3_64(_Hasher): ...
 
 @final
 class xxh3_128(_Hasher): ...
 
-xxh64 = xxh3_64
 xxh128 = xxh3_128
 
 def xxh32_digest(data: _DataType, seed: int = ...) -> bytes: ...