[FIX] base: do not use async reserved keyword

In python 3.7, async is a reserved keyword.
Using it in a method signature produces a syntax error.
Replace by an argument async_load and catch the previous calls in a kw for
backward compatibility.
Log a warning on public methods.

The **kw can be removed in v12 and officially deprecate the 'async' parameter.

Fixes #25578
Closes #25783

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