Ensure empty line before code and math blocks in doxygen pydoc

Sphinx requires an empty line before code and math blocks, whereas
doxygen does not.  This update ensures that a blank line is included
before generated code and math blocks in the pydoc output.  This is
done by post-processing the docstring line by line to check whether
any newlines need to be added.  This way, if the original doxygen
source already includes an empty line before a block, an additional
unnecessary empty line is not added.

Updating the expected test output for doxygen_basic_translate, which
now adds the necessary empty line before the code block.  Adding
further test cases to doxygen_translate_all_tags to explicitly verify
that a newline is added in the pydoc output before both code and math
blocks that appear within a paragraph.

Additionally, empty lines previously appearing at the beginning of the
generated docstrings are now removed.  This does not alter the
behavior of the tests.

Examples/test-suite/python/doxygen_basic_translate_runme.py

@@ -46,6 +46,7 @@ Title: Minuses:
 * it\'s null
 
 Warning: This may not work as expected
+
 .. code-block:: c++
 
     int main() { while(true); }