WIP: python render improvement

[wolfv]

This renders the data more properly in order to extract all the sources. The plan is:

• render the recipe with all variants & original version, extract the sha hashes + URLs
• re-render the recipe with new $version value and try to get new URLs, and obtain new hashes. If that works, we will find + replace the old hashes in the recipe.

Ideally, this will be implemented in rattler build as a command such as rattler-build bump-version ./recipe -m ./variant-config/*.yaml - but for now I am prototyping this here to get it into the bot.