Change to new load extension system.

[tuna2134]

Rationale

Due to a change in the way extensions are loaded in discord.py v2.

Summary of changes made

added a way to load bot.load_extension asynchronously

Checklist

• [ ] This PR changes the jishaku module/cog codebase
  • [ ] These changes add new functionality to the module/cog
  • [ ] These changes fix an issue or bug in the module/cog
  • [ ] I have tested that these changes work on a production bot codebase
  • [ ] I have tested these changes against the CI/CD test suite
  • [ ] I have updated the documentation to reflect these changes
• [ ] This PR changes the CI/CD test suite
  • [ ] I have tested my suite changes are well-formed (all tests can be discovered)
  • [ ] These changes adjust existing test cases
  • [ ] These changes add new test cases
• [x] This PR changes prose (such as the documentation, README or other Markdown/RST documents)
  • [ ] I have proofread my changes for grammar and spelling issues
  • [x] I have tested that any changes regarding Markdown/RST syntax result in a well formed document