STYLE: Use decorator syntax instead of legacy syntax for defining properties in Cython
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

See the [Cython docs](http://cython.readthedocs.io/en/latest/src/userguide/extension_types.html#properties) for a comparison of the decorator syntax and legacy syntax.  I only found a few instances of the legacy syntax; looks like most of the Cython code is already using decorator syntax.