Update parsing of myst notebooks to new (markdown-it based) parser

[chrisjsewell]

Hey @mwouts, first just to clarify this should not be merged just yet. It is an update to use the (currently) pre-release executablebooks/MyST-Parser#123, which utilises markdown-it-py, my shiny new markdown parser 😁

As you can see, it does not affect any of the tests, and thus shouldn't be noticeable on the front-end.

[codecov]

Codecov Report

Merging #473 into master will decrease coverage by 0.03%.
The diff coverage is 96.22%.

@@            Coverage Diff             @@
##           master     #473      +/-   ##
==========================================
- Coverage   98.86%   98.82%   -0.04%     
==========================================
  Files          80       80              
  Lines        7899     7998      +99     
==========================================
+ Hits         7809     7904      +95     
- Misses         90       94       +4     

Impacted Files	Coverage Δ
jupytext/myst.py	97.32% <96.03%> (-1.01%)	⬇️
jupytext/formats.py	97.48% <100.00%> (ø)
tests/test_ipynb_to_myst.py	100.00% <100.00%> (ø)
tests/test_combine.py	97.26% <0.00%> (-2.74%)	⬇️
jupytext/combine.py	100.00% <0.00%> (ø)
tests/test_stringparser.py	100.00% <0.00%> (ø)
jupytext/stringparser.py	95.65% <0.00%> (+0.19%)	⬆️

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[lgtm-com]

This pull request fixes 1 alert when merging 8a8fb3d into c2b3bfb - view on LGTM.com

fixed alerts:

• 1 for Module is imported with 'import' and 'import from'

[mwouts]

Sounds great! Just let me know when you want this to be merged.

[lgtm-com]

This pull request fixes 1 alert when merging 67e1214 into 35552c4 - view on LGTM.com

fixed alerts:

• 1 for Module is imported with 'import' and 'import from'

[lgtm-com]

This pull request fixes 1 alert when merging eed36e3 into 35552c4 - view on LGTM.com

fixed alerts:

• 1 for Module is imported with 'import' and 'import from'

[lgtm-com]

This pull request fixes 1 alert when merging a848520 into 35552c4 - view on LGTM.com

fixed alerts:

• 1 for Module is imported with 'import' and 'import from'

[chrisjsewell]

@mwouts this is ready for review/merge thanks 😄

[mwouts]

Thank you @chrisjsewell , I will take this for the next release, which I hope to finalize next week.

By the way, I occasionally run pylint on the code, you don't mind if I add a few docstrings, etc to myst.py ? I'll open a separate PR for this.

[chrisjsewell]

Yeh that’s no problem at all 👍