ENH: Compatibility with docutils>=0.18 (from support with sphinx>=5)

[AakashGfude]

This PR updates sphinx-exercise to support sphinx5 and by proxy docutils>=0.18.<0.19

fixes https://github.com/executablebooks/sphinx-exercise/issues/59

[mmcky]

Let's update the requirements so we test against sphinx4 and sphinx5

[mmcky]

@AakashGfude I don't think we need to work on supporting python=3.7 as it is not receiving security updates in ~3 months and will be deprecated anyway.

[codecov-commenter]

Codecov Report

Base: 94.10% // Head: 94.25% // Increases project coverage by +0.15% :tada:

Coverage data is based on head (065b9fd) compared to base (f555d72). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #60 +/- ## ========================================== + Coverage 94.10% 94.25% +0.15% ========================================== Files 7 7 Lines 628 627 -1 ========================================== Hits 591 591 + Misses 37 36 -1 ``` | Flag | Coverage Δ | | |---|---|---| | pytests | `94.25% <ø> (+0.15%)` | :arrow_up: | 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. | [Impacted Files](https://codecov.io/gh/executablebooks/sphinx-exercise/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | Coverage Δ | | |---|---|---| | [sphinx\_exercise/\_\_init\_\_.py](https://codecov.io/gh/executablebooks/sphinx-exercise/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks#diff-c3BoaW54X2V4ZXJjaXNlL19faW5pdF9fLnB5) | `95.06% <ø> (ø)` | | | [sphinx\_exercise/directive.py](https://codecov.io/gh/executablebooks/sphinx-exercise/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks#diff-c3BoaW54X2V4ZXJjaXNlL2RpcmVjdGl2ZS5weQ==) | `97.14% <ø> (ø)` | | | [sphinx\_exercise/transforms.py](https://codecov.io/gh/executablebooks/sphinx-exercise/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks#diff-c3BoaW54X2V4ZXJjaXNlL3RyYW5zZm9ybXMucHk=) | `97.32% <ø> (ø)` | | | [sphinx\_exercise/utils.py](https://codecov.io/gh/executablebooks/sphinx-exercise/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks#diff-c3BoaW54X2V4ZXJjaXNlL3V0aWxzLnB5) | `60.00% <0.00%> (+3.75%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks)

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[mmcky]

@AakashGfude can you do a final review and let me know if you think we need any further testing before putting together a new release

[AakashGfude]

@mmcky this looks good to me. I think we can go ahead and merge.

[AakashGfude]

@mmcky I think we still need some testing for sphinx5. give me a few moments,

[AakashGfude]

Looks good to me now @mmcky , should we cut a release?