fix channel backups with old "cryptography" module
fix channel backups with old "cryptography" module

closes #6314

pooler/electrum-ltc,spesmilo/electrum,spesmilo/electrum,spesmilo/electrum,pooler/electrum-ltc,pooler/electrum-ltc,wakiyamap/electrum-mona,wakiyamap/electrum-mona,spesmilo/electrum,pooler/electrum-ltc,wakiyamap/electrum-mona,wakiyamap/electrum-mona