Add async support

[Ousret]

Hello there,

I recently saw your service, and was trying to integrate it within an ASGI micro-service mix with AI for personal usage.
The thing is that you don't have async support, this solves that.

The PR makes some general improvements to the package itself.

In a nutshell:

• Swap Requests for compatible Niquests
• Persist a Session instead of continuously re-do things like TLS handshake
• Add asynchronous classes that mirror exactly the synchronous ones
• Migrate the project metadata to pyproject.toml
• Fix warnings emitted by latest Flake8
• Minor update to the CI
• Bump the version to 3.0.0 (major)

Close #72
Close #58

[marm123]

Thank you for your pull request @Ousret! We'll review the changes and follow up accordingly.