Remove useless import of aiohttp

aiohttp is not a dependency so using zeep without it raise an
ImportError.

Author: cedk

src/zeep/transports.py

@@ -3,7 +3,6 @@
 from contextlib import contextmanager
 from urllib.parse import urlparse
 
-import aiohttp
 import requests
 from requests import Response
 from requests_file import FileAdapter