[shebang] Change /usr/bin/python -> /usr/bin/env python3

This should fix "unusual interpreter" Lintian warnings here:
https://udd.debian.org/lintian/?packages=gpodder

share/gpodder/examples/gpodder_mark_played.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 # Example script that can be used as post-play extension in media players
 #
 # Set the configuration options "audio_played_dbus" and "video_played_dbus"

share/gpodder/extensions/rm_ogg_cover.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 ####
 # 01/2011 Bernd Schlapsi <brot@gmx.info>

share/gpodder/extensions/rockbox_convert2mp4.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 # Requirements: apt-get install python-kaa-metadata  ffmpeg python-dbus
 # To use, copy it as a Python script into ~/.config/gpodder/extensions/rockbox_mp4_convert.py

share/gpodder/extensions/tagging.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 ####
 # 01/2011 Bernd Schlapsi <brot@gmx.info>

src/gpodder/dbusproxy.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #
 # gPodder - A media aggregator and podcast client

src/gpodder/gtkui/widgets.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #
 # gPodder - A media aggregator and podcast client

src/gpodder/minidb.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #
 # gPodder - A media aggregator and podcast client

src/gpodder/my.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #
 # gPodder - A media aggregator and podcast client

src/gpodder/plugins/soundcloud.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #
 # gPodder - A media aggregator and podcast client