TST: accept small error in threaded random test

freebsd and windows change x87 precision mode (fctrl bit 8 and 9) from
extended to double in child threads so the random numbers cannot be
exactly the same from master and child threads.
see gh-4909
Merge pull request #5051 from juliantaylor/thread-test-fix

TST: accept small error in threaded random test
fix typo
Merge pull request #5053 from Gabriel-p/typo-percentile

DOC: fix typo in np.percentile docstring
ENH: add cube root function np.cbrt for real floats

wraps the C99 cube root function cbrt.