refactor: upgrade repo

[jeanbmar]

The repo is currently broken, this MR fixes it:

• Upgrade all dependencies
• Upgrade project structure to comply with Poetry expectations
• Manage all dependencies through Poetry and get rid of requirements.txt
• Manage gpt-migrate execution through Poetry as well using poetry run gpt-migrate
• Use relative path for internal file reading
• Add a maxtokens options to allow models using only 4096 tokens
• Prevent gpt-migrate from crashing when model response content contains Python None
• Prevent gpt-migrate from crashing when the project to convert contains nested directories
• Use openai models by default

[jeanbmar]

Fixing:

• #50
• #48
• #46
• #45
• #43 (use openai as default)
• #42
• #41

[donfn]

Your PR seems to work, thanks a lot! Although I'm not quite familiar with poetry, I had to mv ./src/gpt_migrate/ ./gpt_migrate, because it wouldn't recognize gpt-migrate as an internal package.