cryptography: add signature of Prehashed (#4048)

* cryptography: add signature of Prehashed

* cryptography: modify signature of Prehashed

mark internal attributes as undocumented
add public attribute digest_size

* cryptography: modify signature of Prehashed

drop Optional from argument of __init__

third_party/2and3/cryptography/hazmat/primitives/asymmetric/utils.pyi

@@ -1,6 +1,16 @@
-from typing import Tuple
+from typing import Optional, Tuple
+from cryptography.hazmat.primitives.hashes import HashAlgorithm
+
 
 def decode_dss_signature(signature: bytes) -> Tuple[int, int]: ...
+
+
 def encode_dss_signature(r: int, s: int) -> bytes: ...
 
-class Prehashed(object): ...
+
+class Prehashed(object):
+    _algorithm: HashAlgorithm   # undocumented
+    _digest_size: int           # undocumented
+    def __init__(self, algorithm: HashAlgorithm) -> None: ...
+
+    digest_size: int