Avoid builtins that were removed in Python 3
    
    - `file` was removed in favor of `open`. Switch to the new spelling.
    - `buffer` was removed in favor of a buffer protocol (and memoryview),
      and `reload` was moved to importlib.
      Both are used in py2-only blocks, so just placate PyLint.
    
    https://fedorahosted.org/freeipa/ticket/5623
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>