Fix type mismatch in serialize_container argument

[alexfikl]

Seems like the new version of mypy is looking into the singledispatch arguments a bit more closely. The issue was that

• the base serialize_container took an ArrayContainer (which is an actual class)
• the ndarray version took an ndarray (which is not a subclass of ArrayContainer!)

This just sets it to Any so that it's a proper fallback for all objects.

Fixes #130

[inducer]

Thanks!