ghc 8.10.5 Sphinx error: (unicode error) 'unicodeescape' codec can't decode bytes

ghc 8.10.5 bootstrap build with Python 39 throws the sphinx error: (unicode error) 'unicodeescape' codec can't decode bytes because there's a \u in the LaTeX code in the bootstrap-generated file docs/users_guide/conf.py.

This fix is to use raw strings, as already done in master:

https://gitlab.haskell.org/ghc/ghc/-/blob/master/docs/users_guide/conf.py#L96

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