Encode Markdown cells with a backslash as raw strings in py:percent scripts

[mwouts]

This is a follow-up on #836

[codecov]

Codecov Report

Merging #850 (3ed4b73) into master (e97819f) will decrease coverage by 0.04%.
The diff coverage is 97.40%.

@@            Coverage Diff             @@
##           master     #850      +/-   ##
==========================================
- Coverage   99.11%   99.06%   -0.05%     
==========================================
  Files         106      110       +4     
  Lines       10345    10512     +167     
==========================================
+ Hits        10253    10414     +161     
- Misses         92       98       +6     

Impacted Files	Coverage Δ
jupytext/cell_reader.py	96.45% <93.02%> (-0.41%)	⬇️
jupytext/header.py	98.78% <93.75%> (-0.59%)	⬇️
jupytext/quarto.py	96.66% <96.66%> (ø)
jupytext/cell_to_text.py	97.83% <100.00%> (+0.03%)	⬆️
jupytext/compare.py	98.75% <100.00%> (+0.02%)	⬆️
jupytext/formats.py	98.44% <100.00%> (+<0.01%)	⬆️
jupytext/jupytext.py	98.37% <100.00%> (-0.01%)	⬇️
jupytext/languages.py	100.00% <100.00%> (ø)
jupytext/magics.py	100.00% <100.00%> (ø)
jupytext/version.py	100.00% <100.00%> (ø)
... and 12 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5e96f4d...3ed4b73. Read the comment docs.