Migrate api-engine runtime from EOL Python 3.8 to 3.11

[YoungHypo]

#787 / #788 move the CI lint job to Python 3.11, but the api-engine runtime is still built on Python 3.8. After #788 lands, flake8 runs on 3.11 while the application is actually built, run, and integration-tested on 3.8.

Affected files

• src/api-engine/Dockerfile#L1— FROM python:3.8
• .pre-commit-config.yaml#L7 — Black hook language_version: python3.8
• src/api-engine/requirements.txt— pygraphviz==1.5, psycopg2-binary==2.8.4

Out of scope

• The Cello agent already ships on python:3.13. Unifying to Python 3.13 (would require bumping Django to 4.2.17+); can be a separate follow-up.

[khanak0509]

Hii @YoungHypo!, I was looking into this issue, after changing these files, cello-api-engine crashes at startup with an error in api/common/enums.py (AttributeError: 'dict' object has no attribute 'remove'). It looks like a Python 3.11 compatibility issue where Enum _member_names changed from a list to a dict. I think we should treat attrs._member_names as a dict now and use use .pop("DisplayStrings", None) instead of .remove().
May I go ahead and open a PR that includes these changes?