Fix active cells in the py:percent format

[mwouts]

This PR reproduces the issue documented at #477

[codecov]

Codecov Report

Merging #479 into master will decrease coverage by 0.00%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
- Coverage   98.82%   98.81%   -0.01%     
==========================================
  Files          80       80              
  Lines        8005     8025      +20     
==========================================
+ Hits         7911     7930      +19     
- Misses         94       95       +1     

Impacted Files	Coverage Δ
jupytext/cell_reader.py	96.63% <75.00%> (-0.16%)	⬇️
jupytext/cell_to_text.py	97.31% <100.00%> (ø)
tests/test_active_cells.py	100.00% <100.00%> (ø)

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