Fix test_cli.invoke for old PyYAML versions (such as 3.13, in Debian 10).

Old versions of PyYAML serialize dicts using {} instead of 'key: value'; but {} was interpreted by .format(), causing a KeyError.

Fixes debian build [1]

• [1] https://jenkins.softwareheritage.org/view/Debian%20packages/job/debian/job/packages/job/DSEA/job/gbp-buildpackage/12/console

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Migrated from D3686 (view on Phabricator)