refactor!: drop support for python 3.8, add some more ruff rules

Explanation of changes

• Set the minimum Python version to 3.9 wherever it's checked
• Modernise code by fixing pyupgrade warnings
  • Change all Dict, List etc. annotations to lowercase
  • Move imports used only for type checking under a TYPE_CHECKING block, with the exception of those used by Pydantic models
  • Change assert False in code to raise AssertionError()
  • raise ... from None for custom exceptions that are re-raised from others (please lmk if there are some for which you want a full/double stack trace instead)
  • Remove some unused loop variables (except some cases in which their end-of-loop value is [ab]used)
  • Other minor adjustments
• Add the flake8-bugbear rules while I'm at it
  • Tackle some problematic "gotchas" reported by B rules

Motivation of changes

Because we want to drop Python 3.8 (https://endoflife.date/python).

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Merge checklist

See also merge request guidelines

• Merge request has been reviewed (in-depth by a knowledgeable contributor), and is approved by a project maintainer.
• New code is covered by unit tests (or N/A).
• New code is documented and docstrings use numpydoc format (or N/A).
• New functionality: considered making private instead of extending public API (or N/A).
• Public API changed: added @deprecated and entry in deprecated code suggestions (or N/A).
• Newly added/adjusted documentation and docstrings render properly (or N/A).
• Pipeline fix or dependency update: post in #software-for-developers channel to merge main back in or update local packages (or N/A).
• Tested on hardware (or N/A).
• CHANGELOG.md for breaking changes and AUTHORS.md have been updated (or N/A).
• Update Hardware backends documentation if backend interface change or N/A
• Check whether performance is significantly affected by looking at the Performance metrics results.
• Windows tests in CI pipeline pass (manually triggered by maintainers before merging).
  • Maintainers do not hit Auto-merge, we need to actively check as manual tests do not block pipeline

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For reference, the issues workflow is described in the contribution guidelines.