Merged requested to merge bitbucket/merged-pr-300 into branch/2.5
Created originally on Bitbucket by alexander_maykin (Alexander Schrijver)
Was already merged in Bitbucket before import, marked as merged by the import user
Test for various XML vulnerabilities
Add tests which show that Quadratic blowup and Billion laughs attacks are possible for openpyxl. Add tests which test for the regression of XXE-attacks for local or remote files.
Use defusedxml for lxml and xml.etree.(c)Elementree to guard against the billion laughs attack and quadratic blowup attack.