Update Django requirements to include 4.2.30, fix mypy errors, and release 2.0.1

[VictorTan93]

📝 Description

Summary

Bumps Django from >=4.0.10, <=4.2.27 to >=4.0.10, <=4.2.30 and pins all dev dependencies to exact versions.

Also updates dev tooling:

• ruff 0.15.12
• django-stubs 4.2.7
• mypy 1.7.1
• pytest 9.0.3

Fixes new mypy due to the updated django-stubs

• Combinable is now imported from django.db.models.expressions instead of django.db.models
• _ErrorMessagesT replaced with _ErrorMessagesMapping in the forms stub

Updates noxfile and GitHub Actions CI to test against Django 4.2.30 only (drops 4.0.10 and 4.2.27).

Releases as version 2.0.1.

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✅ Pre-review checklist for the PR author

• I have tested my code locally on my machine
• I have written tests (if appropriate)
• I have added appropriate label to PR

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