util: also run chmod on EPERM

[fsouza]

Writing a test for this one is tricky, because I was seeing the issue
only when the directory being removed is a docker volume, so instead of
getting EACCES we get EPERM.

This is easy to reproduce though. The existing test fails when the
directory being used for the files is a docker volume:

% docker run \
	-v $(mktemp -d):/tmp \
	-v ${PWD}:/src \
	-w /src \
	python:3 \
	bash -c 'pip install -e . && pip install -r requirements-dev.txt && python -m pytest tests/util_test.py'

[fsouza]

@asottile what do you think? I can add a test with a mock, or not add a test. Or do you have any other ideas? x)

[asottile]