BUG: stats: Fixed the calculation in pareto.stats() to avoid RuntimeWarnings

In pareto.stats(), in the calculation of the skewness, a variable 'bt' is
defined as 'bt = extract(b > 3, b)', but 'b' was still used in the
calculation.  This would lead to spurious RuntimeWarnings.
Merge pull request #2550 from WarrenWeckesser/bug-stats-pareto-skewness

BUG: stats: Fixed the calculation in pareto.stats() to avoid RuntimeWarnings