Use context managers to ensure files are closed
    
    There were several places where context managers were not being used and
    files were potentially not being closed.
    
    I started investigating this since the unit tests are not passing if the
    code directory is mounted with SSHFS. The test setUp failed to remove
    directories with ``shutil.rmtree`` with "directory not empty". Looking
    at the directories, there were .fuse_hidden* files so I assumed the
    file handles were being leaked. The tests are still failing so I've
    either missed something in my search or that was not the problem, but I
    thought this would be a good change in any case.
    
    Signed-off-by: Jeremy Cline <jeremy@jcline.org></jeremy@jcline.org>