doc: replace python3 with python in Makefile

In some setups, documentation won't build if python3 is
used in the "python3 _utils/prepare_sphinx_source.py _source"
command.
This commit replaces python3 with python to make it work.

docs/Makefile

@@ -31,7 +31,7 @@ setup:
 	$(POETRY) install
 	$(POETRY) update
 	cp -TLr source $(SOURCEDIR)
-	python3 _utils/prepare_sphinx_source.py _source
+	python _utils/prepare_sphinx_source.py _source
 
 # Clean commands
 .PHONY: pristine