Align python_requires, addition of requirements-dev.txt

[KaranSinghDev]

Hi,
While setting up the environment to test this repo (specifically in a Kaggle environment), I encountered some dependency conflicts during the pip install process [Optional: you could add: "(related to packages like fsspec being incompatible with pre-installed versions)"].
I also noticed the python_requires in setup.py was set to >=3.5, which differs from the README's recommendation of Python 3.7-3.9.

Based on this, I've made two small improvements:

1. Updated python_requires in setup.py: Changed to >=3.7 to better align with the README and common ecosystem versions.
2. Added requirements-dev.txt: This separates development-only dependencies (like pytest, black, pre-commit, Sphinx) from the core ones in requirements.txt. It clarifies setup for contributors (e.g., via pip install -r requirements-dev.txt).

Hope this improves setup clarity and consistency! Amazing repo by the way