Docs build fails because of a sphinx warning

[cqc-alec]

Warning, treated as error:
<unknown>:1:py:data reference target not found: typing.Union
error: Recipe `build` failed on line 15 with exit code 2
error: Recipe `build-strict` failed on line 25 with exit code 2

[cqc-alec]

We have temporarily disabled the strict build in build_and_test.yml. This should be restored when this issue is fixed.

[CalMacCQ]

Not obvious to me what's gone wrong here. It shouldn't be due to a sphinx upgrade as v7.4.7 should be used per the sphinx = "^7.3.7" specifcation in pytket/docs/pyproject.toml

edit: also fails with sphinx v8