Changed Makefile to allow using 'python3' instead of 'python'

Makefile

 prefix = /usr/local
 
+PYTHON = python
+
 all:
 
 clean:
@@ -11,13 +13,13 @@ distclean: clean
 doc: doc/mkexp.pdf
 
 check:
-	python test.py -v
+	$(PYTHON) test.py -v
 
 dist: doc
-	python setup.py sdist
+	$(PYTHON) setup.py sdist
 
 install: doc
-	python setup.py install --prefix=$(prefix)
+	$(PYTHON) setup.py install --prefix=$(prefix)
 
 .PHONY: all clean distclean doc check dist install