Yeah , I agree that including '%:::z' might not be necessary at all . Including '%:z' and '%::z' format specifiers will give an upper edge to Python. Although , if not going with '%::z' or seconds offset, for now only '%:z' can be a valuable addition . 
I have rectified my code, corrected the indentations and have added cases , where it will also handle any strings of form '%:z' and '%::z' i.e. any character other than 'z' also raising exceptions for cases "%:" or "%::", where no character is specified (Invalid string).   
In case , if we only go with '%:z' format specifier , I will remove the code for '%::z'. 
P.S. I have filled the contributor agreement form. Thanks for the awesome help here. :)