Process.open_files() on Linux might swallow AccessDenied

[giampaolo]

...that's because we pre emptively use os.path.islink() which might return False in case we don't have the permission to perform os.stat() against the designated path:

psutil/psutil/_pslinux.py

Line 1035 in ad2624a

if os.path.islink(file):

Instead we should just call os.readlink() directly and "continue" in case of EINVAL (meaning the path is not a link).
By swallowing AD the function may indirectly return an incomplete list of open files.