Comment variables created from magic commands

[mwouts]

Variables created with magic commands like e.g.

result = %sql SELECT * FROM quickdemo WHERE value > 25

should be commented out in the .py representation.

Cf. #781

[codecov]

Codecov Report

Merging #794 (eda17a0) into master (1d63611) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #794      +/-   ##
==========================================
+ Coverage   99.11%   99.13%   +0.01%     
==========================================
  Files         106      106              
  Lines       10100    10147      +47     
==========================================
+ Hits        10011    10059      +48     
+ Misses         89       88       -1     

Impacted Files	Coverage Δ
jupytext/cli.py	96.17% <100.00%> (+0.08%)	⬆️
jupytext/magics.py	100.00% <100.00%> (ø)
tests/test_cli.py	100.00% <100.00%> (ø)
tests/test_escape_magics.py	100.00% <100.00%> (ø)
jupytext/paired_paths.py	98.49% <0.00%> (+0.75%)	⬆️

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