🐛 Allow Script to have custom metaclass

[huchenlei]

Description

Previously the code was checking whether script_class is a class by checking whether it has type as metaclass, but this implicitly disallows Script classes in extensions to have custom metaclass.

This PR change the implementation to use inspect.isclass to perform the check so that Script classes can have custom metaclass.

Original PR that adds the code:
b523019#diff-e4bc1c55946593391fac69500badc7f35ca9efb23ced1064856ce79fd53f29ceR204

Screenshots/videos:

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests