fix: check if 'std-ref' exists before removing from classes (#82)

Fixes ValueError when building LaTeX/PDF output where 'std-ref' class
may not be present in inline node classes. This was causing builds to
fail with 'ValueError: list.remove(x): x not in list'.

The fix adds a conditional check before attempting to remove the class,
making the code more defensive and compatible with different Sphinx
build scenarios.

Fixes issue discovered in QuantEcon/continuous_time_mcs CI build:
https://github.com/QuantEcon/continuous_time_mcs/actions/runs/19413054039/job/55537214078

Author: mmcky

sphinx_exercise/post_transforms.py

@@ -66,7 +66,8 @@ def run(self):
                         # Get Metadata from Inline
                         inline = node.children[0]
                         classes = inline["classes"]
-                        classes.remove("std-ref")
+                        if "std-ref" in classes:
+                            classes.remove("std-ref")
                         classes.append("std-numref")
                         # Construct a Literal Node
                         literal = docutil_nodes.literal()