Update the xxhash and hashlib stubs (#6031)

* `name`, `block_size`, `digest_size` and `digestsize` attributes of hash objects were made read-only.
* It is now a type-checking error to subclass `xxhash` classes, such as `xxhash.xxh32`. Previously it was an error only at runtime.
* `xxhash` functions now accept strings as input and any object with `__index__()` method for `seed` (instead of requiring an integer). They also fail the type checking if no arguments are given.

stdlib/@python2/hashlib.pyi

@@ -3,10 +3,14 @@ from typing import Tuple, Union
 _DataType = Union[str, unicode, bytearray, buffer, memoryview]
 
 class _hash(object):  # This is not actually in the module namespace.
-    name: str
-    block_size: int
-    digest_size: int
-    digestsize: int
+    @property
+    def name(self) -> str: ...
+    @property
+    def block_size(self) -> int: ...
+    @property
+    def digest_size(self) -> int: ...
+    @property
+    def digestsize(self) -> int: ...
     def __init__(self, arg: _DataType = ...) -> None: ...
     def update(self, arg: _DataType) -> None: ...
     def digest(self) -> str: ...