fix: make ClientT covariant

[AstreaTSS]

Pull Request Type

• [ ] Feature addition
• [x] Bugfix
• [ ] Documentation update
• [ ] Code refactor
• [ ] Tests improvement
• [ ] CI/CD pipeline enhancement
• [ ] Other: [Replace with a description]

Description

This PR makes ClientT covariant. This fixes a couple of typehinting issues I noticed, and... well, it makes sense for it to be covariant anyways, considering what it's meant for.

Changes

See description.

Related Issues

Test Scenarios

import interactions as ipy
from interactions.ext import prefixed_commands as prefixed

class MyBot(ipy.Client):
    pass

class MyContext(prefixed.PrefixedContext[MyBot]):
    pass

@prefixed.prefixed_command()
async def test(ctx: MyContext):
    await ctx.reply("Hi!")

bot = MyBot(...)
prefixed.setup(bot, prefixed_context=MyContext)  # here

Python Compatibility

• [x] I've ensured my code works on Python 3.10.x
• [ ] I've ensured my code works on Python 3.11.x

Checklist

• [x] I've run the pre-commit code linter over all edited files
• [x] I've tested my changes on supported Python versions
• [ ] I've added tests for my code, if applicable
• [ ] I've updated / added documentation, where applicable