Add stubs for cryptography's Fernet.[encrypt|decrypt]_at_time() (#4238)

third_party/2and3/cryptography/fernet.pyi

@@ -5,7 +5,12 @@ class InvalidToken(Exception): ...
 class Fernet(object):
     def __init__(self, key: Union[bytes, Text]) -> None: ...
     def decrypt(self, token: bytes, ttl: Optional[int] = ...) -> bytes: ...
+    # decrypt_at_time accepts None ttl at runtime but it's an implementtion detail and it doesn't really
+    # make sense for the client code to use it like that, so the parameter is typed as int as opposed to
+    # Optional[int].
+    def decrypt_at_time(self, token: bytes, ttl: int, current_time: int) -> bytes: ...
     def encrypt(self, data: bytes) -> bytes: ...
+    def encrypt_at_time(self, data: bytes, current_time: int) -> bytes: ...
     def extract_timestamp(self, token: bytes) -> int: ...
     @classmethod
     def generate_key(cls) -> bytes: ...
@@ -13,5 +18,8 @@ class Fernet(object):
 class MultiFernet(object):
     def __init__(self, fernets: List[Fernet]) -> None: ...
     def decrypt(self, token: bytes, ttl: Optional[int] = ...) -> bytes: ...
+    # See a note above on the typing of the ttl parameter.
+    def decrypt_at_time(self, token: bytes, ttl: int, current_time: int) -> bytes: ...
     def encrypt(self, data: bytes) -> bytes: ...
+    def encrypt_at_time(self, data: bytes, current_time: int) -> bytes: ...
     def rotate(self, msg: bytes) -> bytes: ...