tasklet, channel: use tp_finalize instead of tp_dealloc for complex finalisation code

[akruis]

The C-API documentation states: Starting with Python 3.4, it is recommended not to put any complex finalization code in tp_dealloc, and instead use the new tp_finalize type method.

Actually we have complicated finalisation code in the tp_dealloc functions of tasklet and channel.
Task: move the finalisation logic to the tp_finalize slot.

[akruis]

Implemented by pull request #194

[akruis]

Done. I merged pull request #194 manually.