Standardize Python version and pyenv usage in documentation

[BeckettFrey]

Describe the change

I'd like to suggest improving consistency in our Python-related documentation by recommending the use of pyenv and specifying a minimum Python version. From my investigation, it appears Python >=3.9 is required, but I'm not certain if a more specific version is needed. Proposed improvements:

• Update documentation (e.g., README, CONTRIBUTING) to mention pyenv for Python version management.
• Specify Python >=3.9 as the minimum version required, unless a more specific version is known.
• Optionally add a .python-version file to the repository root with 3.9.0 as a reference for pyenv users.

This will help new contributors and users set up a consistent environment and reduce compatibility issues.

Additional context

References:

• pyenv documentation: https://github.com/pyenv/pyenv
• Example usage:

  pyenv install 3.9.0
  pyenv local 3.9.0

If a more precise Python version is identified, documentation and the .python-version file should be updated accordingly.