Revert "Replace assertRaises with assertRaisesRegex in isbn_tests.py"
This reverts commit 4625bef2d3a1dc0c1929cd9cb745666d75211578
due to a lot of tests failing:
https://travis-ci.org/wikimedia/pywikibot/builds/319000537
Change-Id: Ib41b30f1672a131a7cdf53b3b930fa8db2fd3e4c