Clean up warnings in pysss.c
    
    On older versions of the python headers, some arguments used
    'char *' instead of 'const char *', which means that assigning a
    constant string such as "adduser" threw a warning about discarding
    qualifiers.
    
    This patch cleans up most of these warnings in this file. There
    remain several warnings in the sss_local_methods initialization
    that I do not know how to fix.