pkg_resources is an optional dependency

[mwouts]

Closes #323

[codecov]

Codecov Report

Merging #324 into master will increase coverage by 10.12%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master     #324       +/-   ##
===========================================
+ Coverage   89.06%   99.18%   +10.12%     
===========================================
  Files          71       71               
  Lines        6601     6772      +171     
===========================================
+ Hits         5879     6717      +838     
+ Misses        722       55      -667

Impacted Files	Coverage Δ
jupytext/pandoc.py	95.91% <100%> (+52.16%)	⬆️
tests/test_compare.py	100% <0%> (ø)	⬆️
tests/test_ipynb_to_rmd.py	100% <0%> (ø)	⬆️
tests/conftest.py	100% <0%> (ø)	⬆️
tests/test_metadata_filter.py	100% <0%> (ø)	⬆️
tests/test_read_all_py.py	100% <0%> (ø)	⬆️
tests/test_escape_magics.py	100% <0%> (ø)	⬆️
tests/test_preserve_empty_cells.py	100% <0%> (ø)	⬆️
tests/test_trust_notebook.py	100% <0%> (ø)	⬆️
tests/test_ipynb_to_R.py	100% <0%> (ø)	⬆️
... and 30 more

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