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>

scripts/export_import_between_pageservers.py

@@ -214,8 +214,7 @@ class VanillaPostgres(PgProtocol):
         assert not self.running
         self.running = True
 
-        if log_path is None:
-            log_path = os.path.join(self.pgdatadir, "pg.log")
+        log_path = log_path or os.path.join(self.pgdatadir, "pg.log")
 
         self.pg_bin.run_capture(
             ["pg_ctl", "-w", "-D", str(self.pgdatadir), "-l", log_path, "start"]
@@ -396,7 +395,7 @@ def reconstruct_paths(log_dir, pg_bin, base_tar, port: int):
 
                 query = "select relname, pg_relation_filepath(oid) from pg_class"
                 result = vanilla_pg.safe_psql(query, user="cloud_admin", dbname=database)
-                for relname, filepath in result:
+                for _relname, filepath in result:
                     if filepath is not None:
                         if database == "template0copy":
                             # Add all template0copy paths to template0