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I don't think trace.diff is proposed for commit. I see it more as a supporting file for diagnosing the problem.
I see two problems here:
1. Apparently OP's system opens files with encoding set to 'ascii' by default. This is not the case on any of the systems I have access to (OSX and Linux). I will try to reproduce this issue by setting LANG="en_US.ascii".
2. Regrtest attempts to write a no-ascii character into the trace results file. I suspect this comes from test cases that test import from modules with non-ascii name or with non-ascii identifiers.
I am not sure there is anything we need to change here other than possibly skip tests that use non-ascii identifiers of the systems with default encoding set to ascii. I would be +0 on adding errors='replace' or 'backshlashreplace' to the open() call in write_results_file(), but hardcoding encoding="utf-8" is definitely not the right thing to do.