python3Packages.sphinxHook: move to preFixupPhases

If sphinxHook runs before the fixup phase, possibly generated manpages
will get compressed in the fixup phase.

Changed files
+1 -1
pkgs
development
interpreters
python
+1 -1
pkgs/development/interpreters/python/hooks/sphinx-hook.sh
···
runHook postInstallSphinx
}
-
appendToVar preDistPhases buildSphinxPhase installSphinxPhase
+
appendToVar preFixupPhases buildSphinxPhase installSphinxPhase