feat/flit support

[blast-hardcheese]

Why

As we continue to expand into the realm of offering better Python package manager support, we're happy to add flit to upm.

What changed

• Bump BurntSushi/toml to 1.4.0
• Broke out python/poetry.go and python/pip.go to go along with python/flit.go
• Fix a bug with pip requirements.txt additions where we would take a zero-byte requirements.txt and turn it into \ndep instead of dep\n
• Adding QuirksCannotAddRemove to support flit, which does not directly support add or remove. In the future, we can or delete from pyproject.toml

Test plan

• Import https://github.com/mwaskom/seaborn into Replit
• Run pip install flit to get a compatible version of flit
• Run upm install
• Verify that dependencies are installed

[blast-hardcheese]

A workaround was found for flit. This work can be brought back if necessary.