Feature summary:

pywikibot.textlib needs a wikitext parser. Either mwparserfromhell or wikitextparser is supported. But none of them are required via requitements.txt or setup.py. In result Pywikibot user have to install the wikitext parser additionally. An ImportWarning is shown if one of these libraries is missing:

ImportError: 
Pywikibot is missing a MediaWiki markup parser which is necessary.
Please update the required module with either

    pip install "mwparserfromhell>=0.5.0"

or

    pip install "wikitextparser>=0.47.5"

I propose to make mwparserfromhell mandatory because it is preinstalled on Toolforge. In additon commons_information and patrol scripts needs it.

Benefits:
No additional installation is necessary if Pywikibot is installed from Python Package Index or if requirements.txt is used to install dependencies. If both mwparserfromhell and wikitextparser are installed, wikitextparser is used.

See also:
T326452, T106763