Encrypt and decrypt bytes instead of str

key.decrypt_seal cannot handle decrypted binary data because it uses str instead of bytes. For example:

e = pubkey.encrypt_seal(b"Hello\xebHello") # no error
d = privkey.decrypt_seal(e) # error
# UnicodeDecodeError: 'utf-8' codec can't decode byte 0xeb in position 5: invalid continuation byte

It should be possible to encrypt/decrypt binary contents.