Fix python 3.8 runtime type error

[gshuflin]

Problem

In the create method of core/test.py there is a call to issubclass with a value that can in principle be a typing.Union. In Python 3.6 this works fine; however Python 3.8 changed the type of typing.Union in such a way as cause the issubclass call to throw an exception at runtime.

Solution

Add a check to ensure that the value being checked by issubclass is an instance of type to avoid this error.

[Eric-Arellano]

Thanks!