"make test" calls Python with the -E option which ignore the PYTHON*
environment variables, but Python sub-processes aren't necessarily called with the -E options. For example test_displayhook_unencodable in test_cmd_line. This created problems with my own PYTHONSTARTUP script.

I see 2 solutions to this problems:

1. Fix the tests that might be affected by such problems, by adding the -E option everywhere needed.
2. Fix the Makefile and unset all the PYTHON* environment variables in it.

I think that solution #2 is better. Fixing the tests can be done today, but the authors of future tests might not be aware of this issue. By fixing the Makefile we prevent this problem to occur again in the future.