aes.py: support pycryptodome
aes.py: support pycryptodome

'pycryptodome', which is the replacement for the unmaintained
'pycrypto', requires that the mode be passed to Crypto.Cipher.AES.new()
rather than defaulting to ECB mode.  Specify the mode explicitly to be
compatible with both packages.

google/adiantum,google/adiantum,google/adiantum