Here is an updated patch, but it's a work in progress.

Since we are breaking compatibility anyway, I think a larger cleanup is deserved. For example:
- remove old exception aliases
- make return types consistent (for example, newgroups() should returned structured data as list() does)
- use datetime.datetime objects instead of consuming and producing dates and times as 6-character strings

Additional useful features could be added:

- automatic querying of CAPABILITIES on connection
- a higher-level over() method able to parse the response as a list of header dicts (using LIST OVERVIEW.FMT), with appropriately decoded values (thanks email.header.decode_header())
- ...

Also, we should add an internal mock NNTP server to easily test features. Relying on gmane is nice for simple basic tests, but not much more.

Does it sound reasonable?