Bug#908515: urwid interface can't call PAGER and crashes due to AttributeError

[Nis Martensen]

control: tags 908515 patch

> AttributeError: module 'reportbug.ui.urwid_ui' has no attribute 'system'

The pull request addressing #903969 fixes this one, too:
https://salsa.debian.org/reportbug-team/reportbug/merge_requests/4