Fix nbsphinx_assume_equations for newer versions of Sphinx

[gabalafou]

Fixes compatibility issue with newer versions of Sphinx.

See:

• Remove has_equations from the mathematics domain sphinx-doc/sphinx#13044.

[gabalafou]

Somehow I didn't see PR #822 until today. But as noted, it does not work in all cases.

I started seeing un-Jaxified math while working on a different issue in the PyData Sphinx Theme docs. (Specifically, I was trying to figure out why in docs generated by nbsphinx, MathJax is not applied inside an ipywidget Tab.)

This led me to creating this PR.

We can see that with this PR, the math equations get properly rendered in all three of the examples added in #836.

Examples, screenshots

All three examples share one thing in common: there is no other math in the source document apart from the one featured.

Markdown cell with block math equation

Source code:

nbsphinx/doc/gallery/uno-rst.ipynb

Lines 24 to 36 in b743009

	{
	"cell_type": "markdown",
	"metadata": {},
	"source": [
	"This page has a secondary use, though,\n",
	"which is testing whether a single block math equation works\n",
	"(if there is no other math on the same page):\n",
	"\n",
	"\\begin{equation}\n",
	"\\int\\limits_{-\\infty}^\\infty f(x) \\delta(x - x_0) dx = f(x_0).\n",
	"\\end{equation}"
	]
	}

Screenshot of docs page with this PR applied:

Code cell that outputs a Math object

Source code:

nbsphinx/doc/gallery/due-rst.pct.py

Lines 23 to 31 in b743009

	# %% [markdown]
	# This page has a secondary use,
	# which is testing whether a single math output cell works
	# (if there is no other math on the same page):
	# %%
	from IPython.display import Math
	eq = Math(r'\int\limits_{-\infty}^\infty f(x) \delta(x - x_0) dx = f(x_0)')
	eq

Screenshot of docs page with this PR applied:

Markdown cell with inline math

Source code:

nbsphinx/doc/gallery/cinque-rst.ipynb

Lines 24 to 32 in b743009

	{
	"cell_type": "markdown",
	"metadata": {},
	"source": [
	"This page has a secondary use, though,\n",
	"which is testing whether a single inline math equation works\n",
	"(if there is no other math on the same page): $\\text{e}^{i\\pi} = -1$."
	]
	}

Screenshot of docs page with this PR applied:

[gabalafou]

Even though this PR succeeds with the examples added in #836, it's still worth considering the alternative:

app.set_html_assets_policy("always")

This alternative has the benefit that it's straightforward. It could allow us to have a single code path, so long as we don't care whether this feature (forcing MathJax on nbsphinx-converted pages) works on older versions of Sphinx - because html assets policy was added in 4.1.

I also did a search of GitHub repos for the use of html_assets_policy and it didn't seem to me that this is used much (or at all) beyond loading MathJax. The risk, though, is that may change in the future. The other risk is that suddenly loading MathJax on every documentation page may have unintended consequences.

Overall, this PR is a more conservative change that preserves the status quo from before sphinx-doc/sphinx#13044, but comes with the downside of a split code path and a slightly higher cognitive load.

[mgeier]

Thanks for this PR!

Before looking into this, I would like to fix the CI errors, but those are quite annoying, so I don't know when I'll get around to doing that ...