feat: including typehints; tox to manage packages; mypy to static check; remove unnecessary requirements; change circle ci to use tox; run black and isort to format

[pedroimpulcetto]

Hey there, I'm trying to keep this repository alive by implementing Mypy as a static type checker and making some small improvements, not changing the repository core. I hope it looks good 🙏 but feel free to suggest or decline any changes.

🛠️ Adding Mypy for Better Type Checking

A development dependency, Mypy, was added. It checks for type-related errors before you run the code, making it less prone to runtime errors and smoothing the development process.

📦 Dependency Changes in requirements.txt

The dependencies pytest and responses have been removed. These dependencies aren't necessary for the closeio_api package as they are dev-dependencies, so they were added in tox.ini for testing purposes only.

🔄 Updates in .circleci/config.yml

We are now using pip install tox to run tests and Mypy. A new step, 'Mypy', has been introduced, which runs Mypy on the codebase, ensuring type safety.

💡 Type Hints in closeio_api

Several improvements have been made to the closeio_api/__init__.py file. The main changes include adding type hints to variables and function parameters. This enhancement helps catch type errors at an early stage, making the code safer and more maintainable.