#297 Post-Python 3: New Creative Commons license button URL

[Ahasuerus]

biblio/common.py currently uses https://i.creativecommons.org/l/by/4.0/88x31.png as the location of the graphic for the license button. That URL has been permanently moved to https://licensebuttons.net/l/by/4.0/88x31.png , so we will want to update biblio/common.py to reflect the new location.