Bug#904288: fix pygments test failures with Python 3.7

[Matthias Klose]

Package: src:pygments
Version: 2.2.0+dfsg-1
Severity: important
Tags: sid buster patch

patch taken from
https://bitbucket.org/birkenfeld/pygments-main/pull-requests/772/pep-479-raising-stopiteration-from-a/diff#comment-None