Migrate all Python scripts to Python 3.

Most of them are now _mandatory_ P3 scripts, because I'm tired of
maintaining everything to be compatible with both versions.

The current exceptions are gdb.py (which has to live with whatever gdb
gives it), and kh2reg.py (which is actually designed for other people
to use, and some of them might still be stuck on P2 for the moment).

test/numbertheory.py

@@ -1,7 +1,10 @@
+import sys
 import numbers
 import itertools
 import unittest
 
+assert sys.version_info[:2] >= (3,0), "This is Python 3 code"
+
 def invert(a, b):
     "Multiplicative inverse of a mod b. a,b must be coprime."
     A = (a, 1, 0)