build: default PYTHON to python3 in Makefile

So that direct calls to `make` which spawn the configure script do not use Python 2.

PR-URL: #37764
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ash Cripps <acripps@redhat.com>
Reviewed-By: Richard Lau <rlau@redhat.com>

Makefile

@@ -1,7 +1,7 @@
 -include config.mk
 
 BUILDTYPE ?= Release
-PYTHON ?= python
+PYTHON ?= python3
 DESTDIR ?=
 SIGN ?=
 PREFIX ?= /usr/local