ENH: reduce overhead of probs check of choice

Use kahan_sum as multinomial does.
Closes gh-4188
MAINT: regenerate with cython 0.20 and fix a warning it emits

Fixed warning is "Non-trivial type declarators in shared declaration".
DOC: add versionadded:: 1.8.0 to ufunc.at docstring
MAINT: fix some f2py related build warnings.
Merge pull request #4221 from rgommers/fix-f2py-warning

MAINT: fix some f2py related build warnings.
Merge pull request #4220 from juliantaylor/ufunc-at-ver

DOC: add versionadded:: 1.8.0 to ufunc.at docstring
Merge pull request #4213 from juliantaylor/choice-improv

improve random.choice performance and regen with cython 0.20