bashtage/arch: Release 4.9.0

This is a feature and bug release. Removed support for Python 2.7. Added auto_bandwidth to compute optimized bandwidth for a number of common kernel covariance estimators. This code was written by Michael Rabba. Added a parameter rescale to arch_model that allows the estimator to rescale data if it may help parameter estimation. If rescale=True, then the data will be rescaled by a power of 10 (e.g., 10, 100, or 1000) to produce a series with a residual variance between 1 and 1000. The model is then estimated on the rescaled data. The scale is reported ARCHModelResult.scale. If rescale=None, a warning is produced if the data appear to be poorly scaled, but no change of scale is applied. If rescale=False, no scale change is applied and no warning is issued. Fixed a bug when using the BCA bootstrap method where the leave-one-out jackknife used the wrong centering variable. Added ARCHModelResult.optimization_result to simplify checking for convergence of the numerical optimizer. Added random_state argument to HARX.forecast to allow a RandomState objects to be passed in when forecasting when method='bootstrap'. This allows the repeatable forecast to be produced. Fixed a bug in VarianceRatio that used the wrong variance in nonrobust inference with overlapping samples