Fix Ruff typing deprecations (#951)

[sunilkumarvalmiki]

Summary

• Replace the deprecated typing.Dict, typing.List, and typing.Tuple
  annotations with Python 3.9-compatible built-in generics.
• Move Iterator and Sequence imports to collections.abc where required.
• Keep the change limited to the three files reported by issue 18 Deprecation errors from Ruff (use dict instead of Dict, and collections.abc instead of typing, etc.) #951.

Scope

Changed files:

• src/paho/mqtt/client.py
• src/paho/mqtt/publish.py
• tests/debug_helpers.py

The commit contains 14 additions and 14 deletions. It does not change MQTT
runtime behavior, dependencies, workflows, Ruff configuration, pre-commit
configuration, or project metadata.

Before / after

At baseline revision ac779fe09f4797781718fd64f045e27c37b30ce0, the pinned
Ruff 0.1.9 check reported 18 UP006/UP035 diagnostics and exited 1. On this
branch, the same check reports zero diagnostics and exits 0.

Verification

• Ruff 0.1.9: passed
• Pinned pre-commit Ruff hook: passed
• Focused compatibility tests: 2 passed
• Python 3.9 AST parse: passed
• Changed-file compilation: passed
• git diff --check: passed

The aggregate tox -e lint command remains non-green because existing
repository-wide Black, codespell, and Windows-typeshed mypy findings remain;
those unrelated findings are not changed here.

The latest issue discussion says PR #793 need not block this focused change
and that configuration changes should not be proposed. This PR follows that
scope and does not modify or supersede PR #793.

Addresses #951.

[JamesParrott]

LGTM. Exactly what I wanted.

I made a branch based on this PR and ran the checks and tests on my fork:

Many thanks Sunil!