Avoid resource warnings when calling pycman.config.init_with_config()

Config files were previously being closed by the garbage collector. Now they
are explicitly closed when the file is popped from the stack, or when the
function raises an exception.

The “diff --ignore-space-change” option helps you see what actually changed here. Also, let me know if you’d prefer this be sent to a mailing list or something, I wasn’t really sure.