Python 3.13, new default model, cleanups and fixes

• added Python 3.13 support;
• changed default OpenAI model to gpt-4o to benefit from newer model and lower prices;
• limited number of tool function calls to 50 per chat, as gpt-4o-2024-05-13 was observed constantly calling a tool function with the same arguments;
• added more ablated prompters to facilitate evaluation;
• fixed missing handling for when tiktoken is unable to find encoding for model;
• various cleanups;
• added tentative workarounds for Python 3.10 async process runner cleanup bug;