Support Sage notebooks

[mwouts]

Closes #727

[codecov]

Codecov Report

Merging #732 (66b8434) into master (7cabad8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #732   +/-   ##
=======================================
  Coverage   99.10%   99.10%           
=======================================
  Files         104      104           
  Lines        9862     9866    +4     
=======================================
+ Hits         9774     9778    +4     
  Misses         88       88           

Impacted Files	Coverage Δ
jupytext/languages.py	100.00% <ø> (ø)
jupytext/formats.py	98.44% <100.00%> (+<0.01%)	⬆️
tests/test_auto_ext.py	100.00% <100.00%> (ø)

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