PEP 3108: Fix Sphinx warnings

Author: Secrus

peps/pep-3108.rst

@@ -1140,9 +1140,6 @@ References
 .. [#pil] Python Imaging Library (PIL)
     (http://www.pythonware.com/products/pil/)
 
-.. [#twisted] Twisted
-    (http://twistedmatrix.com/trac/)
-
 .. [#irix-retirement] SGI Press Release:
     End of General Availability for MIPS IRIX Products -- December 2006
     (http://www.sgi.com/support/mips_irix.html)