Faster, and offline venvs

[tusharsadhwani]

• Try to find a satisfyable Python version locally before trying to resolve it from GitHub.
• Fix fallback GitHub data not being used properly in Rust (Python venv creation would have worked offline until now as long as the Python version in the fallback github data matches local, Rust didn't, because when the http request failed it would bubble up the error.)
• Use the bundled microvenv with python -m ensurepip for macos and linux, which is much faster than the venv module.