The command line used to run the tests is important.
If you run through "test.regrtest", it should "chdir" to a sandbox directory to run the test. If you call directly the test_bufio.py file, it runs in the current directory (as before the patch).
And the permission of the current directory may give a difference in the 2nd case.
Variants:
python.exe -m test.regrtest test_bufio
python.exe Lib/test/regrtest.py test_bufio
python.exe Lib/test/test_bufio.py (fail if home directory is RO)
And another full run:
python.exe -m test.regrtest
Try some of these variants, it may help diagnose the windows issue.
(Note: maybe some antivirus or other software may be related to the race condition described on #7443)