Make jupytext an optional dependency (#336)

* Add "markdown" extra dependency

* Guard `jupytext` import and raise helpful error

* Remove extra quotes from environment file

* Avoid restricting installation instructions to be specific to `pip`/PyPI

---------

Co-authored-by: Matt Fisher <3608264+mfisher87@users.noreply.github.com>

Author: agriyakhetarpal

dev-environment.yml

@@ -15,5 +15,5 @@ dependencies:
   - black
   - voici
   - pip:
-      - .
+      - .[markdown]
       - jupyterlite-xeus >=2.1.2

jupyterlite_sphinx/jupyterlite_sphinx.py

@@ -28,9 +28,13 @@
     new_code_cell,
 )
 
-import jupytext
 import nbformat
 
+try:
+    import jupytext
+except ImportError:
+    jupytext = None
+
 try:
     import voici
 except ImportError:
@@ -617,6 +621,11 @@ def run(self):
             notebook_is_stripped: bool = self.env.config.strip_tagged_cells
 
             if notebook_path.suffix.lower() == ".md":
+                if jupytext is None:
+                    raise ImportError(
+                        "jupyterlite-sphinx requires the jupytext package to process Markdown notebooks. "
+                        'Install "jupyterlite-sphinx[markdown]" with your package manager of choice.'
+                    )
                 if self._target_is_stale(notebook_path, target_path):
                     nb = jupytext.read(str(notebook_path))
                     if notebook_is_stripped:

pyproject.toml

@@ -17,11 +17,13 @@ dependencies = [
     "jupyter_server",
     "jupyterlab_server",
     "jupyterlite-core >=0.2,<0.8",
-    "jupytext",
     "nbformat",
     "sphinx>=4",
 ]
 
+[project.optional-dependencies]
+markdown = ["jupytext"]
+
 [dependency-groups]
 dev = [
     "hatch",