Fix docutils 0.19 deprecation warnings

[chrisjsewell]

Running the test suite (with tox command) now gives the following warnings:

tests/test_docutils.py: 9 warnings
tests/test_renderers/test_error_reporting.py: 18 warnings
tests/test_renderers/test_fixtures_docutils.py: 89 warnings
tests/test_renderers/test_fixtures_sphinx.py: 155 warnings
tests/test_renderers/test_include_directive.py: 8 warnings
tests/test_renderers/test_myst_config.py: 5 warnings
tests/test_renderers/test_myst_refs.py: 9 warnings
tests/test_sphinx/test_sphinx_builds.py: 57 warnings
  /Users/chrisjsewell/Documents/GitHub/myst-parser/myst_parser/mdit_to_docutils/base.py:55: DeprecationWarning: The frontend.OptionParser class will be replaced by a subclass of argparse.ArgumentParser in Docutils 0.21 or later.
    settings = OptionParser(components=(parser_cls,)).get_default_values()

tests/test_docutils.py: 712 warnings
tests/test_renderers/test_error_reporting.py: 1224 warnings
tests/test_renderers/test_fixtures_docutils.py: 6724 warnings
tests/test_renderers/test_fixtures_sphinx.py: 10540 warnings
tests/test_renderers/test_include_directive.py: 544 warnings
tests/test_renderers/test_myst_config.py: 760 warnings
tests/test_renderers/test_myst_refs.py: 612 warnings
tests/test_sphinx/test_sphinx_builds.py: 3876 warnings
  /Users/chrisjsewell/Documents/GitHub/myst-parser/.tox/py37-sphinx5/lib/python3.7/optparse.py:1000: DeprecationWarning: The frontend.Option class will be removed in Docutils 0.21 or later.
    option = self.option_class(*args, **kwargs)

tests/test_docutils.py::test_cli_latex
  /Users/chrisjsewell/Documents/GitHub/myst-parser/myst_parser/parsers/docutils_.py:265: FutureWarning: The default for the setting "use_latex_citations" will change to "True" in Docutils 1.0.
    _run_cli("latex", "LaTeX documents", argv)

tests/test_docutils.py::test_cli_latex
  /Users/chrisjsewell/Documents/GitHub/myst-parser/myst_parser/parsers/docutils_.py:265: FutureWarning: The default for the setting "legacy_column_widths" will change to "False" in Docutils 1.0.)
    _run_cli("latex", "LaTeX documents", argv)

tests/test_docutils.py::test_help_text
  /Users/chrisjsewell/Documents/GitHub/myst-parser/tests/test_docutils.py:86: DeprecationWarning: The frontend.OptionParser class will be replaced by a subclass of argparse.ArgumentParser in Docutils 0.21 or later.
    OptionParser(components=(Parser,)).print_help(stream)

tests/test_renderers/test_fixtures_docutils.py::test_syntax_extensions[1-dollarmath]
tests/test_renderers/test_fixtures_docutils.py::test_syntax_extensions[31-amsmath]
tests/test_renderers/test_fixtures_docutils.py::test_syntax_extensions[56-deflist]
tests/test_renderers/test_fixtures_docutils.py::test_syntax_extensions[71-fieldlist]
tests/test_renderers/test_fixtures_docutils.py::test_syntax_extensions[85-colon_fence]
tests/test_renderers/test_fixtures_docutils.py::test_syntax_extensions[97-replacements]
tests/test_renderers/test_fixtures_docutils.py::test_syntax_extensions[106-strikethrough]
tests/test_renderers/test_fixtures_docutils.py::test_syntax_extensions[122-tasklist]
  /Users/chrisjsewell/Documents/GitHub/myst-parser/tests/test_renderers/test_fixtures_docutils.py:91: DeprecationWarning: Publisher.setup_option_parser is deprecated, and will be removed in Docutils 0.21.
    option_parser = pub.setup_option_parser()

tests/test_renderers/test_fixtures_docutils.py: 8 warnings
tests/test_renderers/test_myst_config.py: 5 warnings
  /Users/chrisjsewell/Documents/GitHub/myst-parser/.tox/py37-sphinx5/lib/python3.7/site-packages/docutils/core.py:121: DeprecationWarning: The frontend.OptionParser class will be replaced by a subclass of argparse.ArgumentParser in Docutils 0.21 or later.
    usage=usage, description=description)

tests/test_renderers/test_myst_config.py::test_cmdline[1-title-to-header]
tests/test_renderers/test_myst_config.py::test_cmdline[28-linkify]
tests/test_renderers/test_myst_config.py::test_cmdline[38-gfm-strikethrough]
tests/test_renderers/test_myst_config.py::test_cmdline[54-gfm-disallowed-html]
tests/test_renderers/test_myst_config.py::test_cmdline[81-gfm-autolink]
  /Users/chrisjsewell/Documents/GitHub/myst-parser/tests/test_renderers/test_myst_config.py:18: DeprecationWarning: Publisher.setup_option_parser is deprecated, and will be removed in Docutils 0.21.
    option_parser = pub.setup_option_parser()

Perhaps @AA-Turner could suggest any suitable fixes?