Improve dependency links
trydevpi is only added on development version but indeed we can let the choice to the user (with pip option) to use it also for released series.
Also tox.ini set again the link to trydevpi but indeed it could use the option 'process-dependency-links'.
FYI, 'process-dependency-links' is not more deprecated: https://github.com/rtfd/readthedocs.org/pull/3162