close synchronosuly on __del__

[ifrit98]

Misleading Error only occurs in python 3.11:

╭─────────────────────────────── Traceback (most recent call last) ────────────────────────────────╮
│ /example.py:4 in <module>                                                                        │
│                                                                                                  │
│    1 import torch                                                                                │
│    2 import bittensor as bt                                                                      │
│    3 dendrite = bt.dendrite( )                                                                   │
│ ❱  4 raise Exception('Raise random exception.')                                                  │
│    5                                                                                             │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
Exception: Raise random exception.
Exception ignored in: <function dendrite.__del__ at 0x29bcb7600>
Traceback (most recent call last):
  File ".../bittensor/dendrite.py", line 815, in __del__
TypeError: 'NoneType' object is not callable

Actually caused by dendrite.aclosesession not awaited under the hood.

Fixed by simply using synchronous dendrite.closesession() in __del__ desctructor.