Remove the beet executable and update shebangs (#5303)

## Description

This is a very simple PR, just removing the (apparently unused) `beet`
executable from the repository and substituting `bin/env python` for
`bin/python3` where possible.

Fixes #4604.

beet

@@ -1,21 +0,0 @@
-#!/usr/bin/env python
-
-# This file is part of beets.
-# Copyright 2016, Adrian Sampson.
-#
-# Permission is hereby granted, free of charge, to any person obtaining
-# a copy of this software and associated documentation files (the
-# "Software"), to deal in the Software without restriction, including
-# without limitation the rights to use, copy, modify, merge, publish,
-# distribute, sublicense, and/or sell copies of the Software, and to
-# permit persons to whom the Software is furnished to do so, subject to
-# the following conditions:
-#
-# The above copyright notice and this permission notice shall be
-# included in all copies or substantial portions of the Software.
-
-
-import beets.ui
-
-if __name__ == '__main__':
-    beets.ui.main()

docs/changelog.rst

@@ -14,6 +14,7 @@ Bug fixes:
 For packagers:
 
 * The minimum supported Python version is now 3.8.
+* The `beet` script has been removed from the repository.
 
 Other changes:
 

test/rsrc/convert_stub.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """A tiny tool used to test the `convert` plugin. It copies a file and appends
 a specified text tag.

test/testall.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # This file is part of beets.
 # Copyright 2016, Adrian Sampson.