Add support for parsing C++11 final classes

Such as:

  class X final {};

This no longer gives a syntax error.
This change has introduced one more shift-reduce conflict in the parser.
with a conflict with a C style variable declaration with name final:

  class X final;

resulting in a syntax error (for C++ not C). This is an an unusual style
for C++ code and more typical declarations do work:

  X final;

Closes #672

Examples/test-suite/python/final_c_runme.py

@@ -0,0 +1,6 @@
+import final_c
+
+final_c.init()
+f = final_c.cvar.final
+if (f.yval != 123):
+  raise RuntimeError("f.yval fail")