Python 3 support in PL/Python

Behaves more or less unchanged compared to Python 2, but the new language
variant is called plpython3u.  Documentation describing the naming scheme
is included.

src/pl/plpython/expected/plpython_test.out

@@ -1,4 +1,5 @@
 -- first some tests of basic functionality
+CREATE LANGUAGE plpython2u;
 -- really stupid function just to get the module loaded
 CREATE FUNCTION stupid() RETURNS text AS 'return "zarkon"' LANGUAGE plpythonu;
 select stupid();
@@ -7,6 +8,14 @@ select stupid();
  zarkon
 (1 row)
 
+-- check 2/3 versioning
+CREATE FUNCTION stupidn() RETURNS text AS 'return "zarkon"' LANGUAGE plpython2u;
+select stupidn();
+ stupidn 
+---------
+ zarkon
+(1 row)
+
 -- test multiple arguments
 CREATE FUNCTION argument_test_one(u users, a1 text, a2 text) RETURNS text
 	AS