rand_r missing in _windows_flip.pyx error on windows and python 3

[eyaler]

#914 (comment)

copying from the above comment:

On Python3, we need to add 'u'.

Fail(cause rand_r is missing error).

IF UNAME_SYSNAME == "Windows":

Success

IF UNAME_SYSNAME == u"Windows":

see also Python 3.3 + Cython 20.1 build failure and Pull #949