MAINT: special: Use the framework in test_mpmath.py for mpmath-based tests of boxcox and boxcox1p.
Merge pull request #3229 from WarrenWeckesser/fix-boxcox

ENH: special: fix numerical issues in 'boxcox', and add 'boxcox1p'.
ENH: add the limit shape c=0 to genpareto distribution

For c=0, genpareto is equivalent to the exponential
distribution.