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.

scripts/fix-gfx-offsets

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

scripts/fix-map-names

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 #
 # fix-map-names - Fix map names in WAD files
@@ -10,8 +10,6 @@
 # or make target "test-map-names" ("-t" option). Make target "test"
 # ("-t" option) will run this and any other test.
 
-from __future__ import print_function
-
 # Imports
 
 import argparse

scripts/makejson

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

scripts/makepkgs

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

scripts/music-duplicates

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 #
 # Find duplicated music tracks and create a summary report of music

scripts/simplecpp

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