Cell metadata that is specific to text files must come from these files, not ipynb

[mwouts]

Fixes #761

[codecov]

Codecov Report

Merging #775 (268889a) into master (ea7788b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #775   +/-   ##
=======================================
  Coverage   99.11%   99.11%           
=======================================
  Files         106      106           
  Lines       10032    10044   +12     
=======================================
+ Hits         9943     9955   +12     
  Misses         89       89           

Impacted Files	Coverage Δ
jupytext/metadata_filter.py	100.00% <100.00%> (ø)
tests/test_black.py	100.00% <100.00%> (ø)
tests/test_cli.py	100.00% <100.00%> (ø)

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