Upgrade pywikibot to latest version 6.6.2, and add mwparserfromhell

Upgrade pywikibot from 3.0.202007031 to latest version 6.6.2

Last bump was in August 2020 (with 907c6fe).

Also add mwparserfromhell, as a wikitext parser is now
a requirement for pywikibot.

Also change deprecated syntax:

• handleArgs --> handle_args
• pagegenerators.ReferringPageGenerator --> template.getReferences
• ImagePage --> FilePage
• remove test on `page.isEmpty, as the method does not exist anymore (assuming it does not do anything important).
• Accordingly, remove test_addCommonsTemplate_skip_on_empty

Bug: T295238
Change-Id: Iddb2894243320e8b1a25dbfae7d4ffeedaf6b494