Bugzilla has been replaced by Phabricator, and references in the codebase need to be updated. misspelling even recommends that users create bugs in Bugzilla.
$ git grep -i bugzilla
pywikibot/data/api.py: # Bugzilla 49978
pywikibot/families/wikibooks_family.py: 'co': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=28644
pywikibot/families/wikibooks_family.py: 'kn': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=20325
pywikibot/families/wikibooks_family.py: 'sw': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=25170
pywikibot/families/wikibooks_family.py: 'vo': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=37413
pywikibot/families/wikibooks_family.py: 'za': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=20325
pywikibot/families/wikibooks_family.py: 'zu': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=25425
pywikibot/families/wikinews_family.py: 'hu': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=28342
pywikibot/families/wikinews_family.py: 'nl': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=20325
pywikibot/families/wikinews_family.py: 'th': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=28341
pywikibot/families/wikiquote_family.py: 'ang': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=29150
pywikibot/families/wikiquote_family.py: 'ast': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=28964
pywikibot/families/wikiquote_family.py: 'kk': None, # https://bugzilla.wikimedia.org/show_bug.cgi?id=20325
pywikibot/family.py: 'bugzilla': 'bugzilla',
pywikibot/logentries.py: # ( https://bugzilla.wikimedia.org/show_bug.cgi?id=17781 )
pywikibot/site.py: # bug: see https://bugzilla.wikimedia.org/show_bug.cgi?id=7304
pywikibot/site.py: # https://bugzilla.wikimedia.org/show_bug.cgi?id=53894
scripts/cosmetic_changes.py: 39, # Single quotation mark (") - Bugzilla 24093
scripts/misspelling.py: https://bugzilla.wikimedia.org/enter_bug.cgi?product=Pywikibot
tests/link_tests.py: link = Link('bugzilla:1337', source=self.site)
tests/link_tests.py: 'bugzilla:1337 is not a local page on wikipedia:en, and the '
tests/link_tests.py: 'interwiki prefix bugzilla is not supported by PyWikiBot!',
Also these phab tasks need to be reviewed to ensure that no vital information was lost in the migration.