The string type doesn’t support this. You’re probably best off using the regular expression sub method with the re.IGNORECASE option.
>>> import re
>>> insensitive_hippo = re.compile(re.escape('hippo'), re.IGNORECASE)
>>> insensitive_hippo.sub('giraffe', 'I want a hIPpo for my birthday')
'I want a giraffe for my birthday'