python: more linting (#4734)

Ruff has "B" class of lints, including B018 which will nag on useless
expressions, related to #4719. Enable such lints and fix the existing
issues.

Most notably:
- https://beta.ruff.rs/docs/rules/mutable-argument-default/
- https://beta.ruff.rs/docs/rules/assert-false/

---------

Co-authored-by: Alexander Bayandin <alexander@neon.tech>

test_runner/pg_clients/python/pg8000/pg8000_example.py

@@ -6,7 +6,7 @@ import pg8000.dbapi
 
 if __name__ == "__main__":
     kwargs = {
-        k.lstrip("NEON_").lower(): v
+        k.removeprefix("NEON_").lower(): v
         for k in ("NEON_HOST", "NEON_DATABASE", "NEON_USER", "NEON_PASSWORD")
         if (v := os.environ.get(k, None)) is not None
     }