timemodule.c: Cast PyUnicode_AsUTF8() to char* (#1294)

bpo-28769 changed PyUnicode_AsUTF8() return type from const char* to
char* in Python 3.7, but tm_zone field type of the tm structure is
char* on FreeBSD.

Cast PyUnicode_AsUTF8() to char* in gettmarg() to fix the warning:

    Modules/timemodule.c:443:20: warning: assigning to 'char *'
    from 'const char *' discards qualifiers