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.

lumps/genmidi/README.adoc

@@ -121,7 +121,7 @@ to specify a note you want to play. For example, for 'C#' in Octave -3:
 Once you're done, save your changes and type this to build a new GENMIDI
 for testing:
 
-  python mkgenmidi genmidi.lmp
+  python3 mkgenmidi genmidi.lmp
 
 To try it out, get a source port with OPL emulation support (eg. Chocolate
 Doom) and add genmidi.lmp on the command line, eg.
@@ -137,8 +137,7 @@ editors instead of AT2. There's a useful script included that will do
 exactly this. Specify the name of the a2i file and the name of the
 desired output file, as follows:
 
-  python a2i-to-sbi snazzy.a2i snazzy.sbi
+  python3 a2i-to-sbi snazzy.a2i snazzy.sbi
 
 Don't forget to update config.py to use the .sbi version of the file
 instead of the .a2i version!
-

lumps/genmidi/a2i-to-sbi

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 
 import a2i_file

lumps/genmidi/a2i_file.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 #
 # Module to load A2I (AdlibTracker 2) instrument files.

lumps/genmidi/config.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 #
 # Config file for GENMIDI file generation.

lumps/genmidi/dumpgenmidi

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 #
 # Dump the contents of a GENMIDI lump into separate .sbi instrument files,

lumps/genmidi/genmidi.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 #
 # Module for interacting with Doom GENMIDI lumps.

lumps/genmidi/instrument.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 #
 # Module to load OPL instrument files.

lumps/genmidi/midi.py

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

lumps/genmidi/mkgenmidi

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

lumps/genmidi/sbi_file.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: BSD-3-Clause
 #
 # Module to load Creative SBI OPL instrument files.