Adding the follwing line to /etc/portage/package.use works around the bug:
dev-python/click-8.0.2 also fails on my environment, with python_targets_python3_8 and python_targets_python3_9.
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build log (3.8), success
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build log (3.9), fail
O.K, after "emerge =dev-python/sphinx-tabs-3.1.0", and then, "emerge dev-python/click" works fine.
The ebuild should be added DEPEND (or BDEPEND?) with doc? (dev-python/sphinx-tabs).