rstrip doesn't do the same thing as chomp, on so many levels. Read http://perldoc.perl.org/functions/chomp.html and see that chomp is very complex indeed.
However, my main point is that chomp removes at most 1 line ending, whereas rstrip will remove as many as it can.
Here you can see rstrip removing all the newlines:
>>> 'foo\n\n'.rstrip(os.linesep)
'foo'
A much closer approximation of typical Perl chomp usage can be accomplished with re.sub, like this:
>>> re.sub(os.linesep + r'\Z','','foo\n\n')
'foo\n'