Remove call to deprecated importlib find_loader

[smorad]

python3.12 removes find_loader, so replace it so popgym works with python3.12. Also bump flake8 version as old version has broken dependencies.

[codecov-commenter]

Codecov Report

Attention: 116 lines in your changes are missing coverage. Please review.

Comparison is base (e397e5e) 44.39% compared to head (9d54e21) 48.46%.
Report is 23 commits behind head on master.

Files	Patch %	Lines
...opgym/baselines/ray_models/ray_linear_attention.py	0.00%	27 Missing ⚠️
popgym/baselines/ray_models/ray_fwp.py	0.00%	22 Missing ⚠️
popgym/wrappers/flatten.py	75.51%	12 Missing ⚠️
popgym/baselines/models/fwp.py	0.00%	11 Missing ⚠️
popgym/baselines/models/linear_attention.py	0.00%	10 Missing ⚠️
popgym/baselines/ppo.py	0.00%	6 Missing ⚠️
tests/compute_framerate.py	0.00%	4 Missing ⚠️
popgym/envs/minesweeper.py	76.92%	3 Missing ⚠️
popgym/wrappers/discrete_action.py	88.46%	3 Missing ⚠️
popgym/envs/velocity_only_cartpole.py	95.45%	2 Missing ⚠️
... and 15 more

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage   44.39%   48.46%   +4.07%     
==========================================
  Files          72       69       -3     
  Lines        4762     4141     -621     
==========================================
- Hits         2114     2007     -107     
+ Misses       2648     2134     -514     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.