Improve typing on AECEnv.agent_iter

[mfornet]

Description

Minor typing improvement.

for agent in env.agent_iter(): ...

Now agent will be properly typed as AgentId. Address my main (initial) motivation for proposing #1318

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Screenshots

Before & After

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have run pytest -v and no errors are present. (only the same tests that fail on CI, fail locally)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I solved any possible warnings that pytest -v has generated that are related to my code to the best of my knowledge.
• I have added tests that prove my fix is effective or that my feature works (See screenshots)
• New and existing unit tests pass locally with my changes