I've just tried simply adding python3_9 to PYTHON_COMPAT and it seems to be enough - pyqtgraph builds and tests fine for this Python target, both with and without USE=opengl. I can push the change to the tree myself if you're okay with it.
The bug has been closed via the following commit(s): https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=4c93cad59ded1be11f00850464d90b9a0faa1292 commit 4c93cad59ded1be11f00850464d90b9a0faa1292 Author: Marek Szuba <marecki@gentoo.org> AuthorDate: 2020-11-21 22:00:46 +0000 Commit: Marek Szuba <marecki@gentoo.org> CommitDate: 2020-11-21 22:05:26 +0000 dev-python/pyqtgraph: support python3_9 Changed with permission from mgorny on behalf of the Python project. Closes: https://bugs.gentoo.org/754024 Signed-off-by: Marek Szuba <marecki@gentoo.org> dev-python/pyqtgraph/pyqtgraph-0.11.0_rc0.ebuild | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)