dknewkey with ed25519 insecure chmod call / race condition

When generating an ed25519 key with dknewkey a race condition occurs with the file permission.

The problem is in this code:
```
    priv_key = skg.generate()
    with open(private_key_file, 'w') as pkf:
        pkf.write(priv_key.encode(encoder=nacl.encoding.Base64Encoder).decode(">
    if os.name == 'posix':
        os.chmod(private_key_file, 0o600)
    return(priv_key)
```

The chmod is only called after the key is written to the file.

This could be exploited on a multiuser system if keys are generated according to a known scheme. You can test this with fpracer, a proof of concept for such vulnerabilities:
https://github.com/hannob/fpracer

With an unprivileged user account run:
```
./fpracer /etc/dkim/example.com.key
```

With the root account run:
```
mkdir /etc/dkim
cd /etc/dkim
dknewkey --ktype ed25519 example.com
```

To avoid such races it is necessary to already create the file with secure permissions. This can be done for example via os.umask. I have attached a patch.