Python PermissionError on some file operations - windows

Question

I created the following Python 3.5 script:
import sys
from pathlib import Path
def test_unlink():
log = Path('log.csv')
fails = 0
num_tries = int(sys.argv[1])
for i in range(num_tries):
try:
log.write_text('asdfasdfasdfasdfasdfasdfasdfasdf')
with log.open('r') as logfile:
lines = logfile.readlines()
# Check the second line to account for the log file header
assert len(lines) == 1
log.unlink()
not log.exists()
except PermissionError:
sys.stdout.write('! ')
sys.stdout.flush()
fails += 1
assert fails == 0, '{:%}'.format(fails / num_tries)
test_unlink()
and run it like this: python test.py 10000. On Windows 7 Pro 64 with Python 3.5.2, the failure rate is not 0: it is small, but non-zero. Sometimes, it is not even that small: 5%! If you printout the exception, it will be this:
PermissionError: [WinError 5] Access is denied: 'C:\\...\\log.csv'
but it will sometimes occur at the exists(), other times at the write_text(), and I wouldn't be surprised if it happens at the unlink() and the open() too.
Note that the same script, same Python (3.5.2), but on linux (through http://repl.it/), does not have this issue: failure rate is 0.
I realize that a possible workaround could be:
while True:
try: log.unlink()
except PermissionError: pass
else: break
but this is tedious and error prone (several methods on Path instance would need this, easy to forget), and should (IMHO) not be necessary, so I don't think it is practical solution.
So, does anyone have an explanation for this, and a practical workaround, maybe a mode flag somewhere that can be set when Python starts?