Modernize typing

[kdeldycke]

This PR use simpler, ad-hoc native types.

It also defers typing in the same spirit as: csala/mdformat-simple-breaks#5

[csala]

@kdeldycke Allowing the workflow unveiled some problems. I fixed one directly on the github web editor, but now more are showing up. Do you want to take a look? I'm happy to continue on my own, but since the PR was originally yours I feel like overstepping.

It's mostly references that were lost on the conflict resolution, but the web editor is really not user friendly at all to fix these things.

[kdeldycke]

Hmmm. Ah yes, I guess I produce this PR after #20, hence the mixed up...

OK I can fix this one if you prefer.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.50%. Comparing base (93e3972) to head (3b04bd3).

Files with missing lines	Patch %	Lines
mdformat_pyproject/plugin.py	77.77%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
- Coverage   97.29%   92.50%   -4.80%     
==========================================
  Files           2        2              
  Lines          37       40       +3     
==========================================
+ Hits           36       37       +1     
- Misses          1        3       +2     

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[kdeldycke]

OK I just made the last edits to only introduce minimal changes. All tests are passing. I guess you can squash-merge this PR if it's OK to you.

[kdeldycke]

Thanks for the merge! :)