editor: commit to Python 3.

At the time I wrote this editor, it was sensible to try to be Python
2/3 agnostic. P2 is now thoroughly obsolete, so I've removed the P2
affordances. That lets me fix the shebang line to say 'python3', and
chmod the file +x, so that you can run it by its name.

Also, it can be renamed to just 'editor', because now that it's
executable, the fact that it's in Python is nothing but an internal
implementation detail.

editor

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Interactive glyph editor for Bedstead.
 #
@@ -7,20 +7,13 @@
 # Simon Tatham makes this program available under the CC0 Public
 # Domain Dedication.
 
-from __future__ import division, print_function, unicode_literals
-
 import re
 import sys
 import string
-
-try:
-    from tkinter import *
-except ImportError:
-    # Fall back to Python 2 name for module
-    from Tkinter import *    
-
 import subprocess
 
+from tkinter import *
+
 tkroot = Tk()
 
 class Container: