search links broken on readthedocs

[Thomas Gilgenast]

https://github.com/readthedocs/readthedocs.org/issues/6801

a workaround is to downgrade the sphinx version we use on readthedocs

currently we specify the sphinx version in the [docs] extra, so we could add a maximum version there

[Thomas Gilgenast]

we should be tracking https://github.com/readthedocs/readthedocs.org/pull/6804

if this PR is merged we should be able to drop the maximum version constraint

[Thomas Gilgenast]

fixes #78 by adding a maximum version constraint to the sphinx version specified in the [docs] extra

→ <<cset c4933b82dd4c>>

[Thomas Gilgenast]

the PR is now merged and the change is live on readthedocs, so we are free to drop the maximum version constraint going forward