Special handling for python doctest code blocks

A doctest code block begins with ">>>" and is not indented.  Identify
these in doxygen comments and treat them accordingly.  Also add check
to padCodeAndVerbatimBlocks for these because Sphinx requires an empty
line before.

Add test case to doxygen_code_blocks.i.

Examples/test-suite/python/doxygen_code_blocks_runme.py

@@ -50,4 +50,9 @@ A code block for python:
 
 .. code-block:: python
 
-    print('hello world')""")
+    print('hello world')
+
+A python doctest example:
+
+>>> 1 + 1
+2""")