refactor(pubsub): remove EOL Python 3.7/3.8/3.9 false positives and compatibility checks

[chalmerlowe]

Since google-cloud-pubsub requires Python >= 3.10, this PR cleans up obsolete compatibility checks, warnings, and pytest skip conditions targeting Python 3.7, 3.8, and 3.9.

Why this is needed:

• Simplifies the codebase by removing dead compatibility blocks and checks that are always true under Python >= 3.10.
• Cleans up EOL warnings and deprecated import workarounds that are no longer relevant to current runtimes.
• Cleans up unit test decorators to run all OpenTelemetry tests unconditionally.
• Supports clean up efforts to eliminate false positives as found by the version_scanner.
  🦕

[gemini-code-assist]

Code Review

This pull request drops support for Python 3.9, raising the minimum supported version to Python 3.10. This allows for the cleanup of various legacy compatibility checks, imports, and test skips across the codebase. Feedback on the changes suggests that passing cancel_futures=True to self._executor.shutdown() in scheduler.py is redundant because the manual queue draining loop already empties the work queue.