specifically call python3, in scripts

with this change, it's unlikely we'll hit errors again. previously,
some projects used were calling "python" which in context was
python3, but on some setups, the user only has python2 and python3
but no symlink for "python" (which if exists, we assumed linked to
python3)

now it's unambiguous. docs/build/ can probably be updated now, as
a result of this change, to remove the advice about that

resources/grub/patches/0003-specifically-use-python3-in-gentpl.py.patch

@@ -0,0 +1,22 @@
+From cc315e7b4f9e9b7ac4ed2a662526744c51f789d1 Mon Sep 17 00:00:00 2001
+From: Leah Rowe <leah@libreboot.org>
+Date: Sun, 13 Mar 2022 18:08:57 +0000
+Subject: [PATCH 1/1] specifically use python3 in gentpl.py
+
+---
+ gentpl.py | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/gentpl.py b/gentpl.py
+index c86550d4f..cb89cef37 100644
+--- a/gentpl.py
++++ b/gentpl.py
+@@ -1,4 +1,4 @@
+-#! /usr/bin/python
++#! /usr/bin/python3
+ #  GRUB  --  GRand Unified Bootloader
+ #  Copyright (C) 2010,2011,2012,2013  Free Software Foundation, Inc.
+ #
+-- 
+2.25.1
+