chore: drop Python 3.9 support (EOL)

[Zac-HD]

Remove Python 3.9 from CI matrix, setup.cfg classifiers, tox envlist, mergify checks, and bump python_requires to >=3.10.

Inline code that was gated behind sys.version_info checks:

• Use slots=True directly in dataclass decorator (3.10+)
• Use futures.Future[t.Any] directly (3.9+)
• Remove redundant version checks and imports in tests

(extracted from #551)

[jd]

@mergify queue

[mergify]

queue

🟠 Waiting for conditions to match

Details

• -closed [📌 queue requirement]
• -conflict [📌 queue requirement]
• -draft [📌 queue requirement]
• any of: [📌 queue -> configuration change requirements]
  • -mergify-configuration-changed
  • check-success = Configuration changed
• any of: [🔀 queue conditions]
  • all of: [📌 queue conditions of queue default]
    • check-success=test (3.10, py310)
    • check-success=test (3.11, py311)
    • check-success=test (3.12, py312)
    • check-success=test (3.13, py313)
    • check-success=test (3.14, pep8)
    • check-success=test (3.14, py314,py314-trio)
    • any of:
      • #approved-reviews-by >= 1
      • author = dependabot[bot]
      • author = jd

[mergify]

Merge Queue Status

🟠 Waiting for queue conditions

Required conditions to enter a queue

• -closed [📌 queue requirement]
• -conflict [📌 queue requirement]
• -draft [📌 queue requirement]
• any of [📌 queue -> configuration change requirements]:
  • -mergify-configuration-changed
  • check-success = Configuration changed
• any of [🔀 queue conditions]:
  • all of [📌 queue conditions of queue default]:
    • check-success=test (3.10, py310)
    • check-success=test (3.11, py311)
    • check-success=test (3.12, py312)
    • check-success=test (3.13, py313)
    • check-success=test (3.14, pep8)
    • check-success=test (3.14, py314,py314-trio)
    • any of:
      • #approved-reviews-by >= 1
      • author = dependabot[bot]
      • author = jd