ENH: Add metadata for parallel_read_safe = True

[steppi]

Closes #146

This PR adds return {"parallel_read_safe": True} to setup in jupyterlite_sphinx.py, marking jupyterlite_sphinx as parallel read safe in the metadata.

Without this, Sphinx forces a serial read

WARNING: the jupyterlite_sphinx extension does not declare if it is safe for parallel reading,
assuming it isn't - please ask the extension author to check and make it explicit
WARNING: doing serial read

I've determined that the extension is parallel read safe. I think the only point of contention is the use of self.env.temp_data seen below, to avoid regenerating notebooks if a docstring is processed multiple times.

jupyterlite-sphinx/jupyterlite_sphinx/jupyterlite_sphinx.py

Lines 375 to 381 in 09be958

	if "generated_notebooks" not in self.env.temp_data:
	self.env.temp_data["generated_notebooks"] = {}
	directive_key = f"{self.env.docname}-{self.lineno}"
	notebook_unique_name = self.env.temp_data["generated_notebooks"].get(
	directive_key
	)

and later on

jupyterlite-sphinx/jupyterlite_sphinx/jupyterlite_sphinx.py

Lines 408 to 409 in 09be958

	if notebook_unique_name is None:
	nb = examples_to_notebook(self.content, warning_text=warning_text)

Since the keys in the dictionary are unique per appearance of the directive, and the only operations are setting for a key if the key doesn't exist, or checking if the key is in the dictionary, parallel reads should have no impact. I've tested this building SciPy's documentation with parallel reading, and found no issues.