Fix paragraph in paragraph rendering

[jakobandersen]

• Remove use of the six module as Python 3 is required anyway.
• Fix Doxygen \p command results in newline #666. The same problem is present in the Breathe docs at https://breathe.readthedocs.io/en/latest/doxygen.html#_CPPv4NUt16_ex_doxygen_group2N1E where there is a wrong paragraph break after "Also see".
• Fix markup in the Tinyxml example where a verbatim \n was interpreted as a line break due to missing \c.

[vermeeren]

@jakobandersen Thanks!