Since we're using llvm11 by default since bug 1184920, two packages are
unresolvable because libqt5-qttools-doc requires clang7:
openSUSE:Backports:SLE-15-SP3/kdevelop5 and
openSUSE:Backports:SLE-15-SP3/python3-pyside2.

The problem is that due to packaging changes clang7 conflicts with newer Clang
versions, so libqt5-qttools-doc cannot be installed in parallel with them.

The requirement is probably because clang contained the "builtin headers"
before I moved them to libclang in
https://build.opensuse.org/request/show/684104, but this never made it to SLE.

Either we can convince SUSE to bring this change to SLE, or maybe we can relax
the Requires to Recommends? I don't have a better solution right now. I'm
pretty sure kdevelop5 doesn't actually need libqt5-qttools-doc, it's just an
accidental dependency.