fix: move contextlib import to module top level in pool.py

Was imported inline inside two method bodies, violating PEP 8.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqliteclient/pool.py

@@ -1,6 +1,7 @@
 """Connection pooling for dqlite."""
 
 import asyncio
+import contextlib
 from collections.abc import AsyncIterator, Sequence
 from contextlib import asynccontextmanager
 from typing import Any
@@ -94,8 +95,6 @@ async def acquire(self) -> AsyncIterator[DqliteConnection]:
             yield conn
         except BaseException:
             # On error (including cancellation), close connection
-            import contextlib
-
             with contextlib.suppress(BaseException):
                 await conn.close()
             self._in_use.discard(conn)
@@ -139,8 +138,6 @@ async def close(self) -> None:
 
         # Close in-use connections
         for conn in list(self._in_use):
-            import contextlib
-
             with contextlib.suppress(Exception):
                 await conn.close()
         self._in_use.clear()