Incompatible with zope.component 5

[jamadden]

Some long-deprecated backwards-compatibility aliases were removed in zope.component 5. Unfortunately, this package is using some of those deprecated aliases, making it incompatible with zope.component 5:

$ mktmpenv -p `which python2.7`
created virtual environment CPython2.7.18.final.0-64 in 367ms
…
This is a temporary environment. It will be deleted when you run 'deactivate'.
$ pip install -U zope.annotation zope.tales contentratings
...
Installing collected packages: zope.interface, …, contentratings
Successfully installed BTrees-4.7.2...contentratings-1.2.1...zope.component-5.0.0...

$ python -c 'import contentratings.category'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "//python2.7/site-packages/contentratings/category.py", line 15, in <module>
    from contentratings.interfaces import (
  File "//python2.7/site-packages/contentratings/interfaces.py", line 11, in <module>
    from zope.component.interfaces import IObjectEvent
ImportError: cannot import name IObjectEvent

I'd be happy to work up a PR for this if there's a chance it could get merged.