Rolling-quantiles

Fix behavior around NaNs, and publish Python 3.9 versions.

The project is stable enough to warrant a full release.

I introduced more unit tests to expand coverage, and fixed minor bugs that were revealed as a consequence.

Extended the interface to enable linear interpolations between the discrete values approximating a true quantile fraction. The estimation characteristics may be controlled via alpha and beta parameters, each between zero and one.

Binaries available via `pip install rolling-quantiles` for Linux, MacOS, and Windows running Python 3.8+ and NumPy 1.20.