The currency.py uses a fixer.io legacy API (api.fixer.io).
As of March 6th 2018, the legacy fixer API is deprecated, to use the service we need a (free, but registration-bound) fixer access token. So i believe it is needed to replace

url = 'http://api.fixer.io/%7B%7D'.format(when.isoformat())

with

url = 'http://data.fixer.io/api/%7B%7D?access_key=12345678900000000000000000000000'.format(when.isoformat())

where of course 12345678900000000000000000000000 is your personal access key.