Issue 20317: Remove debugging code from contextlib

- Alex J Burke noticed a debugging raise in the commit that
  fixed the original bug reported in issue 20317
- this showed that multiple iterations through the affected
  loop wasn't actually being tested
new plan: just remove typecasts (closes #20374)
Issue #20384: Fix the test_tarfile test on Windows.

On Windows os.open() error message doesn't contain file name.
Fix typo.
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