Bug#963653: libgpuarray: FTBFS with Sphinx 3.1: File "/usr/lib/python3/dist-packages/sphinx/domains/c.py", line 3093, in object_type / raise NotImplementedError()

[Rebecca N. Palmer]

Control: reassign -1 src:sphinx
Control: retitle -1 sphinx breaks python3-breathe < 4.15
Control: tags -1 patch

This does not happen with breathe 4.19 (from experimental); this 
upstream report suggests it happens with breathe < 4.15.

https://github.com/sphinx-doc/sphinx/issues/7424

Hence, a fix for this is to increase the Breaks: version to 4.15
https://salsa.debian.org/python-team/modules/sphinx/-/blob/debian/master/debian/control#L67
and move python3-breathe from experimental to unstable before sphinx.