Improvements to various commands before v2

- jsk sh performs better on Windows (it now uses PowerShell if
  available, or cmd if not, instead of trying to invoke commands
  directly, making it more like the Linux variant)
- jsk pip added as an alias to jsk sh pip
- jsk selftest adjusted to make more sense on intents versions

Make jsk su perform better on bots that disable the GUILD_MEMBERS intent

Add shim for Task.current_task deprecation in JishakuBase.submit

Add guild_subscriptions info to selftest

Push PaginatorInterface.update fix for NotFound to patch version

Fix logout message

Add new WrappedPaginator flag and fix existing behavior wrt regression

Fix some stuff and deprioritize 3.6

Update copyright year, fix some voice-related annoyances and add JISH…

…AKU_NO_DM_TRACEBACK

Change how subclassing the Jishaku cog works