tests: Properly open/close the file
    
    In addition to being just cleaner, this removes the only warning when
    running the tests for the pyrpkg.sources module with Python3.
    
    Not that we're anywhere ready to have pyrpkg run with Python3, but any
    little step in that direction is a good thing.
    
    Plus it means pyrpkg.sources works fine with both Python2 and Python3!