Allow new style classes to be used with jelly.setUnjellyableForClassTree

[njal]

Currently, twisted.spread.jelly.setUnjellyableForClassTree uses the check type(i_) == types.ClassType, which only works for old style classes to determine whether it should check if i_ is a subclass of the given base class.

In order for it to work with new style classes, I've written a patch that makes use of inspect.isclass (which to my knowledge is available back to python 2.1 -- correct me if I'm mistaken here).

[njal]

I've checked that unit tests for twisted.spread.pb and twisted.spread.jelly runs after applying the patch.

[therve]

This is a duplicate of #2950.