tests/cpydiff: Use literal markup for __init__ in exception_construction.

The description and workaround wrapped __init__ in single backticks.
With Sphinx default_role="any", that became an unresolved :any: xref
("reference target not found: __init__"). Use double backticks for
literal code, matching the same test's ``super().__init__`` markup.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: kwagyeman

tests/cpydiff/core_exception_construction.py

@@ -1,8 +1,8 @@
 """
 categories: Core,Exceptions
-description: Throwing a derived exception class instance in its `__init__` without first calling ``super().__init__`` is a TypeError
+description: Throwing a derived exception class instance in its ``__init__`` without first calling ``super().__init__`` is a TypeError
 cause: In MicroPython, an object is incompletely constructed if it does not call its superclass init function or return normally from its ``__init__``. This prevents its usage in some circumstances.
-workaround: Call the superclass `__init__` method before raising the exception.
+workaround: Call the superclass ``__init__`` method before raising the exception.
 """