Unable to install requirements for examples

I'm trying to run `pip install -r examples/requirements.txt`, which is `pip install -r examples/requirements.in` now but keep getting the following error

```
Collecting gym==0.21 (from stable_baselines3->-r requirements.in (line 12))
Using cached gym-0.21.0.tar.gz (1.5 MB)
Preparing metadata (setup.py) ... error
error: subprocess-exited-with-error

× python setup.py egg_info did not run successfully.
│ exit code: 1
╰─> [3 lines of output]
/home/alvaro/anaconda3/envs/meltingPot/lib/python3.11/site-packages/setuptools/_distutils/dist.py:270: UserWarning: Unknown distribution option: 'tests_require'
warnings.warn(msg)
error in gym setup command: 'extras_require' must be a dictionary whose values are strings or lists of strings containing valid project/version requirement specifiers.
[end of output]

note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

× Encountered error while generating package metadata.
╰─> See above for output.

note: This is an issue with the package mentioned above, not pip.
hint: See above for details.
```

I can't run `python self_play_train.py` or ` python sb3_train.py`.