Makefile: Upgrade to Python 3

Python 2 died 1,849 days ago on 1/1/2020.
* Node.js no longer supports its use.
    * nodejs/node#25789 was closed in February 2020.

src/Makefile

@@ -217,7 +217,7 @@ $(KSTAT.NODE) :	LIBS +=		-lkstat
 NODE_SUBDIR_ENV = \
 			V=1 \
 			NODE_DTRACE_PROVIDER_REQUIRE=hard \
-			PYTHON="/opt/local/bin/python2.7" \
+			PYTHON="/opt/local/bin/python3" \
 			CC="$(GCC)" \
 			CXX="$(GXX)" \
 			CPPFLAGS="$(NODE_CPPFLAGS)" \