Fix abc.Iterator import error

Change `import abc` to `from collections import abc` to fix AttributeError
where `abc.Iterator` was not found (standard library abc module doesn't
have Iterator, it's in collections.abc).

Also add cast to fix mypy no-any-return errors.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: laughingman7743

pyathena/polars/result_set.py

@@ -1,8 +1,8 @@
 # -*- coding: utf-8 -*-
 from __future__ import annotations
 
-import abc
 import logging
+from collections import abc
 from multiprocessing import cpu_count
 from typing import (
     TYPE_CHECKING,
@@ -14,6 +14,7 @@
     Optional,
     Tuple,
     Union,
+    cast,
 )
 
 from pyathena import OperationalError
@@ -139,7 +140,7 @@ def as_polars(self) -> "pl.DataFrame":
         """
         import polars as pl
 
-        dfs = list(self)
+        dfs = cast(List["pl.DataFrame"], list(self))
         if not dfs:
             return pl.DataFrame()
         if len(dfs) == 1: