TST: tests for skew/kurtosis vs expect
BUG: explicit formulas for invgamma moments:
n-th moment only exists for a>n
BUG: fix moments of dweibull distribution
BUG: fix moments of R distribution
BUG: fix moments of burr/fisk distributions
MAINT: fix self.b for betaprime distribution
BUG: fix a typo in F distribution skew
MAINT: slightly improve readability of f._stats
BUG: fix the formula for skew for fatiguelife distribution
TST: re-enable several tests for continuous distributions
BUG: fix the 4th moment of randint
BUG: fix moments for nct

I've no idea what exactly was wrong with the prev version of the
explicit formulas in nct._stats; this version is just a retype
from a paper.
WIP: still working on ncf: is it that integration fails @expect?
temporary @ncf
BUG/TST: (partially)fix zipf moments (require a>n+1)

zipf is still problematic: expect does not converge, generates warnings
BUG: fix the expersion for the 4th moment of hypergeom