* builds/meson/*.py: Fix name of python executable for auxiliary scripts.

The previous change to check the return code of `run_command` invocations
caused the CI to fail.  Although most scripts used `python_exe` as the
program command, the script to determine the project version did not.

But, all scripts used `python` as the shebang, and this is not available on
all systems.  Particularly Debian does not provide a `python` command,
though `python3` does exist.  This meant that formerly the version number
was lacking, and now the build simply fails.

Instead, rely on `python3` since it is guaranteed to exist when running
meson, and `python2` is end of life anyway.

builds/meson/extract_freetype_version.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2020-2021 by
 # David Turner, Robert Wilhelm, and Werner Lemberg.

builds/meson/extract_libtool_version.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2020-2021 by
 # David Turner, Robert Wilhelm, and Werner Lemberg.

builds/meson/generate_reference_docs.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2020-2021 by
 # David Turner, Robert Wilhelm, and Werner Lemberg.

builds/meson/parse_modules_cfg.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2020-2021 by
 # David Turner, Robert Wilhelm, and Werner Lemberg.

builds/meson/process_ftoption_h.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2020-2021 by
 # David Turner, Robert Wilhelm, and Werner Lemberg.