Add support for playing Audible AAXC (.aaxc) files [PATCH v4]

The AAXC container format is the same as the (already supported) Audible
AAX format but it uses a different encryption scheme.

Note: audible_key and audible_iv values are variable (per file) and are
externally fed.

It is possible to extend https://github.com/mkb79/Audible to derive the
audible_key and audible_key values.

Relevant code:

def decrypt_voucher(deviceSerialNumber, customerId, deviceType, asin, voucher):
    buf = (deviceType + deviceSerialNumber + customerId + asin).encode("ascii")
    digest = hashlib.sha256(buf).digest()
    key = digest[0:16]
    iv = digest[16:]

    # decrypt "voucher" using AES in CBC mode with no padding
    cipher = AES.new(key, AES.MODE_CBC, iv)
    plaintext = cipher.decrypt(voucher).rstrip(b"\x00")  # improve this!
    return json.loads(plaintext)

The decrypted "voucher" has the required audible_key and audible_iv
values.

Update (Nov-2020): This patch has now been tested by multiple folks -
details at the following URL:

https://github.com/mkb79/Audible/issues/3

Signed-off-by: Vesselin Bontchev <vesselin.bontchev@yandex.com>

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