Fix error in latex (pgf) generation of Graphics objects due to spec changes upstream (matplotlib)

[fwjmath]

This pull request fixes a problem of latex generation of Graphics objects due to upstream spec change.

Symptom: When trying to generate latex for a Graphics object, even simple, sage produces the following error.

sage: latex(line([(0, 0), (1, 1)]))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[2], line 1
----> 1 latex(line([(Integer(0), Integer(0)), (Integer(1), Integer(1))]))

File ~/svn/sage/src/sage/misc/latex.py:922, in LatexCall.__call__(self, x, combine_all)
    893 r"""
    894 Return a :class:`LatexExpr` built out of the argument ``x``.
    895 
   (...)
    919     x 2
    920 """
    921 if has_latex_attr(x):
--> 922     return LatexExpr(x._latex_())
    923 try:
    924     f = latex_table[type(x)]

File ~/svn/sage/src/sage/plot/graphics.py:3440, in Graphics._latex_(self, **kwds)
   3424 """
   3425 Return a string plotting ``self`` with PGF.
   3426 
   (...)
   3437     '%% Creator: Matplotlib, PGF backend...
   3438 """
   3439 tmpfilename = tmp_filename(ext='.pgf')
-> 3440 self.save(filename=tmpfilename, **kwds)
   3441 with open(tmpfilename) as tmpfile:
   3442     latex_list = tmpfile.readlines()

File ~/svn/sage/src/sage/misc/decorators.py:416, in suboptions.__call__.<locals>.wrapper(*args, **kwds)
    412         del kwds[key]
    414 kwds[self.name + "options"] = suboptions
--> 416 return func(*args, **kwds)

File ~/svn/sage/src/sage/plot/graphics.py:3394, in Graphics.save(self, filename, **kwds)
   3390     pgf_options = {
   3391         "pgf.texsystem": latex_implementations[0],
   3392     }
   3393 from matplotlib import rcParams
-> 3394 rcParams.update(pgf_options)
   3395 from matplotlib.backends.backend_pgf import FigureCanvasPgf
   3396 figure.set_canvas(FigureCanvasPgf(figure))

File <frozen _collections_abc>:982, in update(self, other, **kwds)

File ~/svn/sage/local/var/lib/sage/venv-python3.12.5/lib/python3.12/site-packages/matplotlib/__init__.py:774, in RcParams.__setitem__(self, key, val)
    772         cval = self.validate[key](val)
    773     except ValueError as ve:
--> 774         raise ValueError(f"Key {key}: {ve}") from None
    775     self._set(key, cval)
    776 except KeyError as err:

ValueError: Key pgf.preamble: Could not convert ['\\usepackage[utf8x]{inputenc}', '\\usepackage[T1]{fontenc}'] to str

This is due to a change of spec in matplotlib. More precisely, the option pgf.preamble accepts no longer a list of strings, but only a string. This can be seen also in the updated tutorial.

The current pull request resolves this problem. After applying the changes, the previous command produces some pgf output that can be used as a latex presentation of the Graphics object.

I am not sure about how to add test for this change. There are already some tests in the _latex_ function, but somehow it is not executed.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

[fwjmath]

Another slightly off-topic comment: For the preamble, as current latex versions defaults to utf8 encoding, it may be a good idea to revise the current preset preamble, as it may lead to problems in some cases. But I think that this is not urgent at all, as I am not sure about how widely spread this utf8 support currently is.