fix error when building a Generic type

[ppwwyyxx]

fix #515

The added unittest would fail before this PR and now pass. The check if isinstance(fn_or_cls, type) and issubclass(fn_or_cls, Generic) is kept as a workaround for python3.8.

The failure before this PR is:

>       return buildable.__build__(*args, **kwargs)
E       TypeError: Config.__build__() got multiple values for argument 'self'
E
E       Fiddle context: failed to construct or call GenericClass at <root> with positional arguments: (), keyword arguments: (self=1).

[ppwwyyxx]

@dhr would you mind taking a look? Thanks!

[dhr]

Thanks for the fix! Sorry for the slow turnaround.