gh-106690: Add a .coveragerc file to the CPython repository (#8150)

gh-106690: Add a .coveragerc file to the CPython repository (#8150)

The added file is the coverage default at some point in time + checking branches both ways + IDLE additions, labelled as such and somewhat designed to be unlikely to affect other files.  Located in the CPython repository directory, it can be used where it is or copied elsewhere, depending on how one runs coverage.
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Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>