Fix round trip to md with %%python3 cell magic

[mwouts]

Code cells can start with ```python3 in Markdown files. In that case we insert a %%python3 cell magic at the top of the cell.

Closes #365.

[codecov]

Codecov Report

Merging #368 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #368      +/-   ##
==========================================
+ Coverage   99.05%   99.05%   +<.01%     
==========================================
  Files          74       74              
  Lines        7311     7316       +5     
==========================================
+ Hits         7242     7247       +5     
  Misses         69       69

Impacted Files	Coverage Δ
jupytext/cell_reader.py	96.91% <100%> (ø)	⬆️
tests/test_read_simple_markdown.py	100% <100%> (ø)	⬆️

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