The "Jupytext Notebook" viewer works for more notebook extensions

[mwouts]

Closes #803

[codecov]

Codecov Report

Merging #882 (d2d5f43) into main (002fa06) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #882   +/-   ##
=======================================
  Coverage   99.05%   99.05%           
=======================================
  Files         111      111           
  Lines       10633    10633           
=======================================
  Hits        10532    10532           
  Misses        101      101           

Impacted Files	Coverage Δ
jupytext/version.py	100.00% <100.00%> (ø)

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