Document how to fix jupytext stripping mystnb file metadata. #1247

using either a local metadata filter, or a Jupytext configuration file

tests/functional/round_trip/test_myst_header.py

@@ -0,0 +1,58 @@
+from jupytext import reads, writes
+from jupytext.compare import compare
+from jupytext.config import load_jupytext_configuration_file
+
+
+def test_myst_header_is_stable_1247_using_inline_filter(
+    md="""---
+jupytext:
+  formats: md:myst
+  notebook_metadata_filter: -jupytext.text_representation.jupytext_version,settings,mystnb
+  text_representation:
+    extension: .md
+    format_name: myst
+    format_version: 0.13
+kernelspec:
+  display_name: Python 3 (ipykernel)
+  language: python
+  name: python3
+mystnb:
+  execution_mode: 'off'
+settings:
+  output_matplotlib_strings: remove
+---
+""",
+):
+    nb = reads(md, fmt="md")
+    md2 = writes(nb, fmt="md")
+
+    compare(md2, md)
+
+
+def test_myst_header_is_stable_1247_using_config(
+    jupytext_toml_content="""notebook_metadata_filter = "-jupytext.text_representation.jupytext_version,settings,mystnb"
+""",
+    md="""---
+jupytext:
+  formats: md:myst
+  text_representation:
+    extension: .md
+    format_name: myst
+    format_version: 0.13
+kernelspec:
+  display_name: Python 3 (ipykernel)
+  language: python
+  name: python3
+mystnb:
+  execution_mode: 'off'
+settings:
+  output_matplotlib_strings: remove
+---
+""",
+):
+    config = load_jupytext_configuration_file("jupytext.toml", jupytext_toml_content)
+
+    nb = reads(md, fmt="md", config=config)
+    md2 = writes(nb, fmt="md", config=config)
+
+    compare(md2, md)