Replace Optional type annotations with | None

[dgw]

What it says on the tin. We've talked about this a few times in PR reviews, and on IRC; I kicked it off in #2642 and decided we could use a tracking issue for the rest. Transformed from tree -I __pycache__ sopel:

• bot.py
• builtins
  • __init__.py
  • admin.py
  • adminchannel.py
  • announce.py
  • calc.py
  • choose.py
  • clock.py
  • countdown.py
  • currency.py
  • dice.py
  • emoticons.py
  • find.py
  • find_updates.py
  • help.py
  • invite.py
  • isup.py
  • lmgtfy.py
  • ping.py
  • pronouns.py
  • rand.py
  • reload.py
  • safety.py
  • search.py
  • seen.py
  • tell.py
  • tld.py
  • translate.py
  • unicode_info.py
  • units.py
  • uptime.py
  • url.py
  • version.py
  • wikipedia.py
  • wiktionary.py
  • xkcd.py
• cli
  • __init__.py
  • config.py
  • plugins.py
  • run.py
  • utils.py
• config
  • __init__.py
  • core_section.py
  • types.py
• coretasks.py
• db.py
• formatting.py
• irc
  • __init__.py: irc: versionadded notes + Optional annotation migration #2642
  • abstract_backends.py
  • backends.py
  • capabilities.py
  • isupport.py
  • modes.py
  • utils.py
• lifecycle.py
• loader.py
• logger.py
• module.py
• modules
• plugin.py
• plugins
  • __init__.py
  • capabilities.py
  • exceptions.py
  • handlers.py
  • jobs.py
  • rules.py
• privileges.py
• tests
  • __init__.py
  • factories.py
  • mocks.py
  • pytest_plugin.py
• tools
  • __init__.py
  • _events.py
  • calculation.py
  • identifiers.py
  • jobs.py
  • memories.py
  • target.py
  • time.py
  • web.py
• trigger.py

Note that the 8.1.0 milestone is just a starting point. We don't have to do all of this; it's only tagged for visibility.