This is because the way matplotlib's setupext.py has changed the detection of qhull compared to previous versions. The message from matplotlib about qhull is
qhull: yes [pkg-config information for 'libqhull' could not
be found. Using local copy.]
So matplotlib is using pkg-config to detect qhull but our current qhull ebuild doesn't install any .pc files. It is possibly a debian addition.
It looks like fedora used to produce .pc file for qhull but they have stopped to include them around fedora 25.
There are .pc.in files on the github repo (in the build folder) but for some reason they are not included in the tarball we have, and according to time stamps they should have been. qhull's README.txt encourage packagers to distribute .pc files but no tools is given to produce them.
I think matplotlib erred on that change to qhull detection.
I note that matplotlib master branch (3+) has just decided to vendor qhull:
# Qhull doesn't distribute pkg-config info, so we have no way of
# knowing whether a system install is recent enough. Thus, always use
# the vendored version.
But the 2.2.x maintenance branch is still looking for .pc file.