Provide backwards compatibility with Python 2.3 where 'set()' is not …

…a builtin datatype.

This should correct the majority of the errors in this test run:
	http://hudson.communitycheetah.org/job/Cheetah%20(next)/label=Cheetah,python=python23/lastBuild/testReport/

Signed-off-by: R. Tyler Ballance <tyler@slide.com>

src/Template.py

@@ -41,6 +41,12 @@ def acquire(self):
         def release(self): 
             pass
 
+try:
+    x = set()
+except NameError:
+    # Python 2.3 compatibility
+    from sets import Set as set
+
 from Cheetah.Version import convertVersionStringToTuple, MinCompatibleVersionTuple
 from Cheetah.Version import MinCompatibleVersion
 # Base classes for Template