fix: case where not all futures were rejected

[markmarkmarkthebest]

When debugging aiormq in pycharm, pycharm patches asyncio module. It changes _asyncio.Future to a different object then asyncio.Future.
By changing the condition in FutureStore.reject_all from isinstance(future, asyncio.Future) to asyncio.isfuture(future), it will also be true for the _asyncio.Future object when debugging in pycharm. The asyncio.isfuture, checks that the given object is compatible to asyncio.Future.
https://docs.python.org/3/library/asyncio-future.html#asyncio.isfuture

The problem happened to me in ubuntu 20.04, python 3.8.10, pycharm 2024.1.3 professional.