support/scripts/pkg-stats: fix Python 3.8 deprecation warning

support/scripts/pkg-stats: fix Python 3.8 deprecation warning

With Python 3.8, the following deprecation warnings are emitted:

/home/thomas/projets/buildroot/./support/scripts/pkg-stats:418: DeprecationWarning: The explicit passing of coroutine objects to asyncio.wait() is deprecated since Python 3.8, and scheduled for removal in Python 3.11.

/home/thomas/projets/buildroot/./support/scripts/pkg-stats:536: DeprecationWarning: The explicit passing of coroutine objects to asyncio.wait() is deprecated since Python 3.8, and scheduled for removal in Python 3.11.

The correct way to pass coroutines is to use asyncio.create_task(),
but this is rather new method (Python 3.7), and using it breaks
compatibility with older Python versions. As suggested at
https://docs.python.org/3/library/asyncio-task.html#asyncio.create_task,
use the more cryptic, but also more compatible asyncio.ensure_future()
method.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>