Cipher.update_into can corrupt memory if passed an immutable python object as the outbuf

Previously, `Cipher.update_into` would accept Python objects which implement the buffer protocol, but provide only immutable buffers: ```pycon >>> outbuf = b"\x00" * 32 >>> c = ciphers.Cipher(AES(b"\x00" * 32), modes.ECB()).encryptor() >>> c.update_into(b"\x00" * 16, outbuf) 16 >>> outbuf b'\xdc\x95\xc0x\xa2@\x89\x89\xadH\xa2\x14\x92\x84 \x87\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00' ``` This would allow immutable objects (such as `bytes`) to be mutated, thus violating fundamental rules of Python. This is a soundness bug -- it allows programmers to misuse an API, it cannot be exploited by attacker controlled data alone. This now correctly raises an exception. This issue has been present since `update_into` was originally introduced in cryptography 1.8.

Quoted source text, attributed separately from HOL analysis.