pandoc 3.0 compatablity

[mroeschke]

xref #886

pandoc removed the deprecated --atx-headers option in 3.0 https://pandoc.org/releases.html#pandoc-3.0-2023-01-18, and since this is hardcoded in the conversion juptext will fail with pandoc 3.0

Unknown option --atx-headers.
Try pandoc --help for more information.
Traceback (most recent call last):
...
  File "...lib/python3.10/site-packages/jupytext/pandoc.py", line 92, in md_to_notebook
    pandoc(
  File "...python3.10/site-packages/jupytext/pandoc.py", line 35, in pandoc
    raise PandocError(
jupytext.pandoc.PandocError: pandoc exited with return code 6

[mwouts]

Thank you @mroeschke for reporting this. I see #1099 that should solve this issue - I'll keep you posted.

[mwouts]

I have merged the linked PR, and I will release jupytext==1.15.0 soon

[mroeschke]

Thanks for the quick fix!