Update Python version constraint in setup.py to >=3.8 for dependency compatibility

[chigwell]

User Story
As a software developer,
I want to update the Python version constraint in setup.py to >=3.8
so that the application remains compatible with modern dependencies and avoids runtime errors caused by unsupported Python 3.6 features.

Background
The current setup.py specifies python_requires='>=3.6', but newer dependencies like langchain-openai (v0.1.2) and langchain-mistralai (v0.1.1) may require Python 3.8+ features. This mismatch could lead to installation failures or silent runtime errors in environments using Python 3.6-3.7. The tests/test_beatrica.py and core functionality rely on these dependencies, creating technical debt and compatibility risks with CI/CD pipelines and contributor environments.

Acceptance Criteria

• Modify python_requires in setup.py from >=3.6 to >=3.8
• Verify compatibility of langchain-openai and langchain-mistralai with Python 3.8+
• Update documentation in README.md to remove references to Python 3.6 support
• Validate installation in clean Python 3.8/3.9/3.10 virtual environments
• Ensure all tests in tests/test_beatrica.py pass under Python 3.8+
• Confirm CI/CD pipeline configurations enforce Python 3.8+ requirement