Add sphinx-3 patch to blender-2.83.4-r1 ebuild

Newer versions of blender, including the currently unstable version in
the main portage tree, have a simple patch to fix documentation
compilation with sphinx-3.  That commit applies unmodified (some lines
get shifted) to blender-2.83.4-r1, so just include it directly to get
blender-2.83.4-r1 to compile with sphinx-3.  It was tested and appears
to work with sphinx-2.4.4 as well (with and without the patch).

diff --git a/media-gfx/blender/blender-2.83.4-r1.ebuild b/media-gfx/blender/blender-2.83.4-r1.ebuild
index 0123456..0123456 100644
--- a/media-gfx/blender/blender-2.83.4-r1.ebuild
+++ b/media-gfx/blender/blender-2.83.4-r1.ebuild
@@ -114,6 +114,11 @@ BDEPEND="
 	nls? ( sys-devel/gettext )
 "
 
+PATCHES=(
+	# Fix compilation with sphinx-3 (ported from newer versions).
+	"${FILESDIR}/${P}-sphinx3.patch"
+)
+
 blender_check_requirements() {
 	[[ ${MERGE_TYPE} != binary ]] && use openmp && tc-check-openmp