SCons: Drop support for Python 2

We now require SCons 3.0+ (first version with Python 3 support),
and we set min required Python 3 version to 3.5 (3.4 and earlier are
EOL).

doc/Makefile

@@ -13,13 +13,6 @@ doxygen:
 	mkdir -p $(OUTPUTDIR)/doxygen
 	doxygen Doxyfile
 
-markdown:
-	rm -rf $(OUTPUTDIR)/markdown
-	mkdir -p $(OUTPUTDIR)/markdown
-	pushd $(OUTPUTDIR)/markdown
-	python2 $(TOOLSDIR)/makemd.py $(CLASSES)
-	popd
-
 rst:
 	rm -rf $(OUTPUTDIR)/rst
 	mkdir -p $(OUTPUTDIR)/rst