Improve doc of IncrementalEncoder
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`getstate` and `setstate` are instance methods just like `encode` and `reset`, but currently they look like static/class methods [in the docs](https://docs.python.org/3/library/codecs.html#codecs.IncrementalEncoder), which is slightly confusing:
<img width="1884" alt="screen shot 2017-07-18 at 10 39 47 am" src="https://user-images.githubusercontent.com/4149852/28322970-8118150c-6ba5-11e7-8d57-598c1b79639e.png">