Delete python 2 branches from third-party stubs (#7741)

Since #7703, we no longer have third-party stubs that support Python 2, so code like `if sys.version_info >= (3, 0)` can be simplified.

stubs/xxhash/xxhash.pyi

@@ -1,12 +1,7 @@
-import sys
 from _typeshed import ReadableBuffer
+from hashlib import _Hash
 from typing_extensions import SupportsIndex, final
 
-if sys.version_info >= (3, 0):
-    from hashlib import _Hash
-else:
-    from hashlib import _hash as _Hash
-
 VERSION: str
 XXHASH_VERSION: str