🔧 Fix docutils deprecation in option parsing

[agoose77]

This PR fixes the deprecation warnings raised by docutils' deprecation of OptionParser.

The help handling is a little ugly - we could introduce a switch on the docutils version like we do for the default settings. However, I think this approach will be less likely to break once the breaking change is introduced.

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Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (b445a0c) 90.08% compared to head (80e4b6a) 90.04%.

Files	Patch %	Lines
myst_parser/mdit_to_docutils/base.py	66.66%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #842 +/- ## ========================================== - Coverage 90.08% 90.04% -0.05% ========================================== Files 24 24 Lines 3360 3364 +4 ========================================== + Hits 3027 3029 +2 - Misses 333 335 +2 ``` | [Flag](https://app.codecov.io/gh/executablebooks/MyST-Parser/pull/842/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | Coverage Δ | | |---|---|---| | [pytests](https://app.codecov.io/gh/executablebooks/MyST-Parser/pull/842/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | `90.04% <66.66%> (-0.05%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks#carryforward-flags-in-the-pull-request-comment) to find out more.

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[chrisjsewell]

Cheers @agoose77!

Note some suggestions on a fix were given here: https://github.com/executablebooks/MyST-Parser/issues/612#issuecomment-1265047994, maybe you want to double check that against your changes here?

Will have a proper look later

[agoose77]

Thanks @chrisjsewell — I actually used that issue as a starting point. As such, I'm hopeful that this should fall in line with the recommended changes where possible.

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