Add flake8 extensions used in our other projects

[GhostofGoes]

Summary

• Adds five flake8 extensions that are used in other pydis projects
• Fix ordering of imports to pass linting
• Add precommit pipenv script (the dependency is already there, but the script isn't for some reason)

[sco1]

This should add flake8-docstrings as described in: https://github.com/python-discord/organisation/issues/53

[sco1]

Never mind, didn't see the other PR that I commented on within the last hour...