Update Python.gitignore to include Streamlit secrets

[arnaudmiribel]

Reasons for making this change:

Want to add .streamlit/secrets.toml in .gitignore as this file is intended to contain sensitive data (credentials, notably!).

Links to documentation supporting these rule changes:

https://docs.streamlit.io/streamlit-cloud/get-started/deploy-an-app/connect-to-data-sources/secrets-management#develop-locally-with-secrets

Link to application or project’s homepage: https://docs.streamlit.io/