fix '--keep-going' sphinx argument error

[julian-poidevin]

109 added --keep-going option but when used the getattr function call in cli.py was creating this error callback :

Traceback (most recent call last):
  File ".venv/bin/sphinx-autobuild", line 8, in <module>
    sys.exit(main())
  File "/workspaces/comboard-linux-image/docs/.venv/src/sphinx-autobuild/src/sphinx_autobuild/cli.py", line 176, in main
    build_args, pre_build_commands = _get_build_args(args)
  File "/workspaces/comboard-linux-image/docs/.venv/src/sphinx-autobuild/src/sphinx_autobuild/cli.py", line 21, in _get_build_args
    val = getattr(args, arg)
AttributeError: 'Namespace' object has no attribute '-keep-going'

This MR fixes that bug by converting the args namespace to a dictonnary and using arg_dict.get instead.

I'm not sure how clean is this solution since it's no longer using argparse getattr function. I'm not a full time Python dev so feel free to change the code.

For more context, here's my original issue comment post : https://github.com/executablebooks/sphinx-autobuild/pull/109#issuecomment-1643891064

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