#!/usr/bin/python3

import re

txt = "This is a test! \n  \n\t\n"

print('Expected:', [txt.rstrip() + "\n"])

# Note that adding a ,1 substitution limit works around the issue.
txt = re.sub(r'(\s*)$', "\n", txt)
print('Actual  :', [txt])

# Beginning in 3.7, this outputs:
# Expected: ['This is a test!\n']
# Actual  : ['This is a test!\n\n']