opkg-utils: Switch to python3

Switch all py script shebangs to `#!/usr/bin/env python3`.

Testing: Successfully ran the following on an IPK feed in $PWD:
 * opkg-make-index --checksum md5 --checksum sha256 -p Packages -m -l Packages.filelist .
 * opkg-graph-deps ./Packages
 * opkg-compare-indexes ./ ./
 * opkg-list-fields font-alias_1.0.3-r2.3.204_all.ipk
 * opkg-show-deps -p Packages font-alias

I'm not sure how to correctly run opkg-update-index and opkg-unbuild.
I attempted to run both with `-h`, no parse errors.

My motivation for this change is to add xz support to opkg filelists,
which requires python3's version of tarfile. See previous commits in
this series for details. However, this is probably a good change in
general since python2 support is starting to dwindle.

Signed-off-by: Haris Okanovic <haris.okanovic@ni.com>
Signed-off-by: Alejandro del Castillo <alejandro.delcastillo@ni.com>

opkg-compare-indexes

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0-only
 from __future__ import absolute_import
 from __future__ import print_function

opkg-graph-deps

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0-only
 from __future__ import absolute_import
 from __future__ import print_function

opkg-list-fields

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0-only
 from __future__ import absolute_import
 from __future__ import print_function

opkg-make-index

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0-only
 """
    Utility to create opkg compatible indexes

opkg-show-deps

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0-only
 from __future__ import absolute_import
 from __future__ import print_function

opkg-unbuild

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0-only
 from __future__ import absolute_import
 from __future__ import print_function

opkg-update-index

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0-only
 from __future__ import absolute_import
 

opkg.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0-or-later
 #   Copyright (C) 2001 Alexander S. Guy <a7r@andern.org>
 #                      Andern Research Labs