Prevent asyncio DeprecationWarning (#1291)

* Prevent asyncio DeprecationWarning

Under some circumstances (see
<https://github.com/xonsh/xonsh/issues/3907>) a DeprecationWarning would
be printed. This is because of passing coroutine objects directly to
asyncio.wait(), which is confusing behaviour that has been deprecated.

<https://github.com/python/cpython/pull/9543> gives information on how
to refactor code that uses the deprecated method.

Signed-off-by: Carmen Bianca Bakker <carmen@carmenbianca.eu>