When validating, check if issubclass of type instead of just type

[jmansilla]

I have so code that should work both in python 2 and 3, and because of that I'm using instead of strings

future.builtins.str

the type of such a thing is

future.builtins.types.newstr.BaseNewStr

and because of that, instead of validating the type is used as a callable.

[sjakobi]

This seems to be fixed. Please close this.

[jmansilla]

Thanks