Now that we combine coverage data across our CI jobs (issue #4261),
this patch removes around a hundred "# pragma: no cover" comments
which are no longer needed, and expands our 100%-coverage requirement
to several previously-omitted files.
The "explain" phase no longer reports lines inside the standard
library, which were occasionally included in (and made for misleading)
explanations.
The constants-collection feature introduced in version 6.131.1 now
also collects the float components of complex literals like "3.5j".
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