Adopt abstract base class syntax

[stevenbird]

NLTK has plenty of abstract base classes. Now that Python has support for these, we need to audit the codebase and adopt this syntax.

cf https://docs.python.org/3.0/whatsnew/2.6.html#pep-3119

[stevenbird]

Thanks @mrecachinas and @kmike for #1223. It looks like we have a model that can be applied across the toolkit now.

[souravsingh]

@stevenbird Is the issue fixed?

[gmotzespina]

@stevenbird it seems that this issue has been fixed, it might be useful to close it.

[stevenbird]

Thanks @souravsingh, @gmotzespina!