isrcsubmit-cdrdao.py: The DiscId class is deprecated (since python-discid 0.5.0)

Running isrcsubmit with a valid username and password (I didn't actually try that when reporting the last issue..) with python-discid >= 0.5.0:
```
Password:
./isrcsubmit-cdrdao.py:49: DeprecationWarning: The DiscId class is deprecated.
Use read/put on module level or Disc:
  with discid.DiscId() as disc:
```

The background is some refactoring that splits `DiscId` up into `Disc` and `Track`, which both are used transparently without additional initialization. `read()` returns a `Disc` object now and is available on module level.

The reason this doesn't break directly is because it is easier to deprecate classes than module level constants. I will remove the `DiscId` in future releases though.