Add __main__ to allow calling via python -m

[matthew-brett]

Allows e.g.

python -m jupytext my_notebook.ipynb --to Rmd

I think this is the correct way to do this, but I don't personally
have much experience with this machinery.

[mwouts]

Thank you @matthew-brett , yes sure that would be an interesting feature! Personally I sometimes call pip or ipykernel in this way, maybe I'll have a look at their implementation... Also sorry for the thresholds on coverage, I know they are set quite high, anyway in this case it should be easy to copy and adjust one of the tests in test_cli.py.

[matthew-brett]

Yes - I use pip that way often too - I basically copied / adapted what pip did, here.

I look into tests fairly soon.

[matthew-brett]

Actually, looking into it, I don't think it will be trivial to get the test coverage up. The test_cli.py module tests the functions in the cli.py module, but the __main__.py here will only run as __main__ - i.e. when the file gets invoked as a script. I think that will have to be tested as a subprocess - but I don't think that will register in the test coverage.

[matthew-brett]

I stand corrected - it looks like the subprocess test did increase the coverage.

[mwouts]

Thank you @matthew-brett for this! Yes indeed the calls through subprocess seem to count in the coverage... except when we're doing fancy things like calling jupytext through pre-commit hooks like in test_pre_commit_3_sync_black_nbstripout.py 😄