Polyagamma

- This beta release adds functions to compute the density and distribution function (cdf) of the Polya-Gamma distribution.

New release with the `scipy.special.logsumexp` updates.

- Adds ability to import sampling functions into third party cython modules.
- pins numpy version of building from `sdist` to `1.19.0` to avoid numpy version incompatibilities.

- A bugfix release that addresses numerical instabilities present in v1.1.0 for the alternate method.
- The minimum numpy version required for the build process is corrected.
- Several other minor improvements.

- All methods runtime speed have been improved significantly, especially the `saddle` method.
- A bug resulting when Passing a sequence for the shape parameter when methods `devroye` and `alternate` methods are specified has been fixed.
- Python 3.6 support has been dropped, a la NEP29.
- Most C-level function have been re-written to improve performance and to conform to the modern c99 standard.