use python3 only for building

Python 2 is very near end-of-life, and Python3-compatible changes to a
few scripts introduced compatibility problems with 2.7 again.  It went
unnoticed for me since my system symlinks "python" to "python3", but
it broke the build on systems where that symlink is still python2.  At
this point in time, I feel it is worth targetting modern Python and
forgetting about 2.7.

graphics/text/create_caption

@@ -1,8 +1,7 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # encoding: utf-8
 # SPDX-License-Identifier: BSD-3-Clause
 
-from __future__ import unicode_literals
 from PIL import Image, ImageFont, ImageDraw
 import sys
 import os

graphics/text/image_dimensions.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 
 import re

graphics/text/rotate

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # SPDX-License-Identifier: BSD-3-Clause
 

graphics/text/smtextgen

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 #
 # Script to generate text graphics using the "small" (HUD) font.

graphics/text/textgen

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 #
 # Script to generate menu and intermission screen 'text' graphics for

graphics/text/tint.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # SPDX-License-Identifier: MIT
 # Copyright (c) 2017 Martin Miller, Nick Zatkovich