Utilities/Sphinx: Avoid deprecated docutils error handling helpers

Merged Brad King requested to merge brad.king/cmake:docutils-error-helper-deprecation into master

The SafeString and ErrorString classes are deprecated and will be removed in Docutils 0.21 or later. They help avoid encoding errors during exception handling in Python <= 2.7, but these days we always use Python 3 to build the documentation, at least during development, when error cases are most likely to occur.

Fixes: #24039 (closed)

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