add async context manager support for the client

[ika2kki]

Description

this PR defines __aenter__ and __aexit__ for the client to support async context manager usage~ state is cleaned up properly after exit. quick example:

import asyncio
import mystbin

async def main():
    async with mystbin.Client() as client:
        paste = await client.create_paste(filename="secret.txt", content="**** ***")

asyncio.run(main())

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
  • I have updated the changelog with a quick recap of my changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)
• I have read and agree to the Developer Certificate of Origin for this contribution

[AbstractUmbra]

Lgtm, thanks!

[ika2kki]

thankies!