Polyagamma

- Extends patch 85 to other modules.

- Fixes a type casting bug that occurs when passing the scale parameter `h` as a numpy array when the method is explicitly set to `devroye`. See 85
- Fixes a bug with the `devroye` method hanging unexpectedly in some corner cases. Details at 83

This minor release re-instates support for python 3.6. Changes include:
- The samplers have improved performance when generating multiple samples using scalar parameter values for `h` and `z`. The speed boost using the default parameter values (`h=1`, `z=0`) are:

`devroye`: 8.67% boost
`alternate`: 50% boost
`saddle`: 159% boost
`gamma`: 0% boost

See https://github.com/zoj613/polyagamma/pull/76 for more details.
- The hybrid sampler has thus been updated based on the benchmarks after 76 was merged.

- This release adds functions to compute the density and distribution function (cdf) of the Polya-Gamma distribution as well as their logarithms.
- Adds type annotations for the python functions.
- Samplers have been optimized a bit and gained a 10-20% speed boost.

- Unifies the interface of distribution functions.