+ allow cmath.sin(z=1) syntax

In the main:
$ ./python -m timeit -r11 -s 'from cmath import sin;z=1j' 'sin(z)'
1000000 loops, best of 11: 312 nsec per loop

With patch:
$ ./python -m timeit -r11 -s 'from cmath import sin;z=1j' 'sin(z)'
1000000 loops, best of 11: 330 nsec per loop

Author: skirpichev