--set-formats fails if old metadata present

[goerz]

Consider the notebook at https://raw.githubusercontent.com/qucontrol/krotov/3ff0bc3b9a4d591ec699a6b3e92ba41f7fd10bc3/docs/notebooks/02_example_lambda_system_rwa_complex_pulse.ipynb

It contains a metadata blob that the older version of jupytext added when editing the file in vim with jupytext.vim:

"jupytext": {
   "main_language": "python",
   "text_representation": {
    "extension": ".md",
    "format_name": "markdown",
    "format_version": "1.0",
    "jupytext_version": "0.8.6"
 }

If I run

jupytext --set-formats ipynb,py:light 02_example_lambda_system_rwa_complex_pulse.ipynb`

I get

[jupytext] Reading 02_example_lambda_system_rwa_complex_pulse.ipynb
[jupytext] Updating notebook metadata with {'jupytext': {'formats': 'ipynb,py:light'}}
[jupytext] Error: Notebook path '02_example_lambda_system_rwa_complex_pulse.ipynb' was expected to have extension '.md'

and the notebook file is not updated with new metadata. It works if I remove the 'jupytext metadata from the ipynb file manually, before running the command.

Do I understand the current README correctly, that "jupytext": {"formats": "ipynb,py"}, is the only metadata that jupytext v.1.0.0 will put in the notebook? If so, it should probably remove the legacy metadata from the notebook.

[mwouts]

Do I understand the current README correctly, that "jupytext": {"formats": "ipynb,py"}, is the only metadata that jupytext v.1.0.0 will put in the notebook? If so, it should probably remove the legacy metadata from the notebook.

Yes, that's the idea. Actually this is almost working, but not yet completely true on version 1.0.0-rc1, so I'll add one more test to cover and fix this...

[mwouts]

@goerz, I have just published 1.0.0-rc2:

pip install jupytext --upgrade --pre

I think we're getting closer to 1.0.0. Can you confirm it solves this issue? Thanks