Python/aqmp: fix type definitions for mypy 0.920

Python/aqmp: fix type definitions for mypy 0.920

0.920 (Released 2021-12-15) is not entirely happy with the
way that I was defining _FutureT:

qemu/aqmp/protocol.py:601: error: Item "object" of the upper bound
"Optional[Future[Any]]" of type variable "_FutureT" has no attribute
"done"

Update it with something a little mechanically simpler that works better
across a wider array of mypy versions.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 20220110191349.1841027-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>