PyO3 0.16.3

This release contains a number of non-breaking additions and fixes since PyO3 0.16.2.

They are mostly centered on improving support for various build configurations. There are also usability tweaks to the #[pyclass] macro and a new intern! macro to create statically-backed PyString objects as an optimization technique.

Thank you to the following users for the improvements:

@adamreichold
@aganders3
@alex
@kmp1
@mejrs
@messense
@mityax
@momirza
@ravenexp
@zh-jq