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.