MAINT: replace internal calls to `norm.pdf` with _norm_pdf
to avoid the public function call overhead. Closes gh-1710
MAINT: avoid nans in logistic pdf
MAINT: avoid nans in pdfs of several distribution
TST: stop silencing the floating-point warnings
changing the doc strings of measurements.sum
correcting the measurements.sum() doc strings
small changes in the description of class hypergeom_gen
TST: tests for skew/kurtosis vs expect
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