Switch from setup.py to pyproject.toml with Poetry for package management

[sibocw]

Description

Switch from setup.py to pyproject.toml with Poetry for package management.

Does this address any currently open issues?

[list open issues here]

[sibocw]

Note: PyTest first finds all the files and functions that start with test and start importing stuff, and only then does it check if the tests should be included based on the command line input. I'm not sure exactly how this works but here's the problem:

1. On Macs with Intel chips, we must not run the unit tests in the provided examples because PyTorch doesn't support the hardware.
2. Therefore, on these runners, we should only run basic tests that are under flygym/tests.
3. But PyTest tries to import stuff from the examples anyway.

I fixed it by specifically asking it to run tests only in a single file: flygym/tests/test_basic.py. This worked. This also didn't work. I'm just going to remove the test now.