Python magic commands may be indented

[mwouts]

Fix #694

[codecov]

Codecov Report

Merging #717 (ba37bac) into master (12fb00e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #717   +/-   ##
=======================================
  Coverage   99.12%   99.12%           
=======================================
  Files          95       95           
  Lines        9356     9359    +3     
=======================================
+ Hits         9274     9277    +3     
  Misses         82       82           

Impacted Files	Coverage Δ
jupytext/magics.py	100.00% <ø> (ø)
tests/test_escape_magics.py	100.00% <100.00%> (ø)

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