flake8 cleanup of ./electrum

I'm guessing you import ecdsa and aes here just to get the error messages, but I think you have these same checks elsewhere so I don't think you need them in ./electrum

Running `flake8 --max-line-length=300 ./electrum` now outputs far fewer things :)

```
./electrum:29:1: F401 'ecdsa' imported but unused
./electrum:34:1: F401 'aes' imported but unused
./electrum:51:1: F403 'from electrum import *' used; unable to detect undefined names
```