Compat with 3.12, drop 3.6

[almarklein]

I saw some errors like this:

Exception ignored in: <function AsyncItemDB.__del__ at 0x103c65300>
Traceback (most recent call last):
  File "/Users/almar/dev/py/itemdb/itemdb.py", line 631, in __del__
    future = self._loop.create_future()

Not sure from what version this is happening (saw it on 3.11). This PR fixes that in drops 3.6 because the fix uses ascyncio.get_running_loop() and that's only available from 3.7.