In mkrel, build documentation with python3

After commit 9583023, the
documentation cannot be built with Python 2.  Run make with
"PYTHON=python3" to ensure that we use Python 3.

src/util/mkrel

@@ -183,9 +183,9 @@ rm -rf $reldir/.git || true
 if test $dodoc = t; then
 	echo "Building doc..."
 	(cd $reldir/src/doc && make -f Makefile.in \
-	    top_srcdir=.. srcdir=. SPHINX_ARGS=-W PYTHON=python html pdf)
+	    top_srcdir=.. srcdir=. SPHINX_ARGS=-W PYTHON=python3 html pdf)
 	(cd $reldir/src/doc && make -f Makefile.in \
-	    top_srcdir=.. srcdir=. SPHINX_ARGS=-W PYTHON=python clean)
+	    top_srcdir=.. srcdir=. SPHINX_ARGS=-W PYTHON=python3 clean)
 fi
 
 echo "Generating tarfiles..."