Use Python 3

The scripts work with Python 3 now (with a few changes).
Remove Python 2 requirement.

build.py

@@ -90,7 +90,7 @@ for pattern in prefixed_fonts:
       copy_character_glyphs=COPY_CHARACTER_GLYPHS,
       scale_character_glyphs_threshold=SCALE_CHARACTER_GLYPHS_THRESHOLD)
 
-for pattern,name in renamed_fonts.iteritems():
+for pattern,name in renamed_fonts.items():
   files = glob(pattern)
   if not files:
     print("Error: pattern '%s' didn't match any files." % pattern)

ligaturize.py

@@ -9,8 +9,6 @@
 #
 # See ligatures.py for a list of all the ligatures that will be copied.
 
-from __future__ import print_function
-
 import fontforge
 import psMat
 import os
@@ -24,16 +22,6 @@ COPYRIGHT = '''
 Programming ligatures added by Ilya Skriblovsky from FiraCode
 FiraCode Copyright (c) 2015 by Nikita Prokopov'''
 
-VERSION_MISMATCH = '''
-Ligaturizer only supports Python 2.x due to a bug in Fontforge:
-  https://github.com/fontforge/fontforge/issues/3057
-You will need to install Fontforge for Python 2 to use this script.
-'''
-
-if sys.version_info[0] != 2:
-    print(VERSION_MISMATCH)
-    sys.exit(1)
-
 def get_ligature_source(fontname):
     # Become case-insensitive
     fontname = fontname.lower()

stat.py

@@ -9,8 +9,6 @@
 #
 # See ligatures.py for a list of all the ligatures that will be copied.
 
-from __future__ import print_function
-
 import fontforge
 import os
 from os import path