1.3.4: `recommonmark` is deprecated

[kloczek]

From https://github.com/rtfd/recommonmark/

Warning: recommonmark is now deprecated. We recommend using MyST for a docutils bridge going forward. See readthedocs/recommonmark#221 for background and discussion.

[tkloczko@pers-jacek mergedeep-1.3.4]$ grep -r recommonmark
docs/source/conf.py:from recommonmark.parser import CommonMarkParser
docs/source/conf.py:from recommonmark.transform import AutoStructify
docs/source/conf.py:        "recommonmark_config",