fix missing positional argument: mode

[AlexChien]

when run command

steempy addkey

and input proper wif key and have wallet unlocked, it throws

new() missing 1 required positional argument: 'mode'

fixed by recommendation found at https://steemit.com/steemdev/@anthonyadavisii/steem-python-bip38-wallet-error-with-fix

[economicstudio]

Hope someone will merge this. I still use official library for certain uses. And one of the first things that I need to do manually is this.

[roadscape]

Thanks @AlexChien! Continued in local #277