refactor(testing-and-utils): Remove unused code and fix style issues

refactor(testing-and-utils): Remove unused code and fix style issues

This PR includes several small refactorings and style fixes aimed at improving code cleanliness, primarily within the test suite and tooling.

Key changes:

*   Removed assignments to unused variables in various test files (`test_dialect.py`, `test_reflection.py`, `test_select.py`).
*   Removed an unused variable in the pytest plugin (`pytestplugin.py`).
*   Removed an unused variable in the topological sort utility (`topological.py`).
*   Fixed a minor style issue (removed an extra blank line) in the `cython_imports.py` script.

Closes: #12539
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12539
Pull-request-sha: 837c1e6cb17f0ff31444d5161329c318b52e48e7

Change-Id: Ifa37fb956bc3cacd31967f08bdaa4254e16911c2