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)