core, frontends: fix invalid translatable strings. Some f-strings where used in translatable text, this has been fixed by using explicit `format()` call (using a script based on `tokenize`). As tokenize messes with spaces, a reformating tool (`black`) has been applied to some files afterwards.

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−these certificates are used for demo purpose only. Those must be the same as in
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−backend_e2e (they are generated there)